azure: fix building in MinGW via Bash

Azure Pipelines supports bash tasks on Windows hosts due to it always
having Git for Windows included. To support this, the Git for Window
directory is added to the PATH environment to make the bash shell
available for execution. Unfortunately, this breaks CMake with the MinGW
generator, as it has sanity checks to verify that no bash executable is
in the PATH. So we can either remove Git for Windows from the path, but
then we're unable to execute bash jobs. Or we can add it to the path,
but then we're unable to execute CMake with the MinGW generator.

Let's re-model how we set the PATH environment. Instead of setting up
PATH for the complete build job, we now set a variable "BUILD_PATH" for
the job. This variable is only being used when executing CMake so that
it encounters a sanitizied PATH environment without GfW's bash shell.

azure-pipelines.yml

@@ -99,8 +99,8 @@ jobs:
   - template: azure-pipelines/powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw64\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;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)'
@@ -115,8 +115,8 @@ jobs:
   - template: azure-pipelines/powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw32\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;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/build.sh

@@ -9,7 +9,9 @@ set -e
 
 SOURCE_DIR=${SOURCE_DIR:-$( cd "$( dirname "${BASH_SOURCE[0]}" )" && dirname $( pwd ) )}
 BUILD_DIR=$(pwd)
+BUILD_PATH=${BUILD_PATH:=$PATH}
 CC=${CC:-cc}
+CMAKE=$(which cmake)
 
 indent() { sed "s/^/    /"; }
 
@@ -31,7 +33,7 @@ echo "Kernel version:"
 uname -a 2>&1 | indent
 
 echo "CMake version:"
-cmake --version 2>&1 | indent
+env PATH="$BUILD_PATH" "$CMAKE" --version 2>&1 | indent
 echo "Compiler version:"
 $CC --version 2>&1 | indent
 echo ""
@@ -41,11 +43,11 @@ echo "## Configuring build environment"
 echo "##############################################################################"
 
 echo cmake ${SOURCE_DIR} -DENABLE_WERROR=ON -DBUILD_EXAMPLES=ON -DBUILD_FUZZERS=ON -DUSE_STANDALONE_FUZZERS=ON ${CMAKE_OPTIONS}
-cmake ${SOURCE_DIR} -DENABLE_WERROR=ON -DBUILD_EXAMPLES=ON -DBUILD_FUZZERS=ON -DUSE_STANDALONE_FUZZERS=ON ${CMAKE_OPTIONS}
+env PATH="$BUILD_PATH" "$CMAKE" ${SOURCE_DIR} -DENABLE_WERROR=ON -DBUILD_EXAMPLES=ON -DBUILD_FUZZERS=ON -DUSE_STANDALONE_FUZZERS=ON ${CMAKE_OPTIONS}
 
 echo ""
 echo "##############################################################################"
 echo "## Building libgit2"
 echo "##############################################################################"
 
-cmake --build .
+env PATH="$BUILD_PATH" "$CMAKE" --build .

azure-pipelines/nightly.yml

@@ -102,8 +102,8 @@ jobs:
   - template: powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw64\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;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
@@ -119,8 +119,8 @@ jobs:
   - template: powershell.yml
     parameters:
       environmentVariables:
+        BUILD_PATH: $(Agent.TempDirectory)\mingw32\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;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