azure: only override PATH when building

We currently unconditionally override the PATH variable with a custom
path with the main intent of making available our own custom MinGW
installation. This worked quite well so far, but is heavily dependent on
the machine we're running this on. And naturally, it fails on the new
Windows machines we need to upgrade to, as tools like CMake are not
contained in the path we currently set up.

Fix this by remodeling the way we set up the PATH environment. Instead
of overriding it completely, we now override it only when executing
the CMake build.

azure-pipelines.yml

@@ -107,8 +107,8 @@ jobs:
   - template: azure-pipelines/powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw64\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
         CMAKE_OPTIONS: -G"MinGW Makefiles" -DDEPRECATE_HARD=ON
-        PATH: $(Agent.TempDirectory)\mingw64\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
 
 - job: windows_mingw_x86
   displayName: 'Windows (x86; MinGW)'
@@ -124,8 +124,8 @@ jobs:
   - template: azure-pipelines/powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw32\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
         CMAKE_OPTIONS: -G"MinGW Makefiles" -DDEPRECATE_HARD=ON
-        PATH: $(Agent.TempDirectory)\mingw32\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
 
 - job: documentation
   displayName: 'Generate Documentation'

azure-pipelines/nightly.yml

@@ -110,8 +110,8 @@ jobs:
   - template: powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw64\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
         CMAKE_OPTIONS: -G"MinGW Makefiles" -DDEPRECATE_HARD=ON
-        PATH: $(Agent.TempDirectory)\mingw64\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
         RUN_INVASIVE_TESTS: true
 
 - job: windows_mingw_x86
@@ -128,8 +128,8 @@ jobs:
   - template: powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw32\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
         CMAKE_OPTIONS: -G"MinGW Makefiles" -DDEPRECATE_HARD=ON
-        PATH: $(Agent.TempDirectory)\mingw32\bin;C:\ProgramData\Oracle\Java\javapath;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\CMake\bin
         RUN_INVASIVE_TESTS: true
 
 - job: linux_x86_bionic_gcc_openssl

ci/build.ps1

@@ -6,6 +6,12 @@ $PSDefaultParameterValues['*:ErrorAction'] = 'Stop'
 if ($Env:SOURCE_DIR) { $SourceDirectory = $Env:SOURCE_DIR } else { $SourceDirectory = Split-Path (Split-Path $MyInvocation.MyCommand.Path -Parent) -Parent }
 $BuildDirectory = $(Get-Location).Path
 
+# Resolve CMake before modifying PATH
+$CMAKE = (Get-Command cmake).Path
+
+# Prepend BUILD_PATH to make available our own tools
+if ($Env:BUILD_PATH) { $Env:PATH = $Env:BUILD_PATH }
+
 Write-Host "Source directory: ${SourceDirectory}"
 Write-Host "Build directory:  ${BuildDirectory}"
 Write-Host ""
@@ -18,7 +24,7 @@ Write-Host "####################################################################
 Write-Host "## Configuring build environment"
 Write-Host "##############################################################################"
 
-Invoke-Expression "cmake ${SourceDirectory} -DBUILD_EXAMPLES=ON ${Env:CMAKE_OPTIONS}"
+Invoke-Expression "& '${CMAKE}' ${SourceDirectory} -DBUILD_EXAMPLES=ON ${Env:CMAKE_OPTIONS}"
 if ($LastExitCode -ne 0) { [Environment]::Exit($LastExitCode) }
 
 Write-Host ""
@@ -26,5 +32,5 @@ Write-Host "####################################################################
 Write-Host "## Building libgit2"
 Write-Host "##############################################################################"
 
-cmake --build .
+Invoke-Expression "& '${CMAKE}' --build ."
 if ($LastExitCode -ne 0) { [Environment]::Exit($LastExitCode) }