azure pipelines: use bundled zlib

Azure Pipelines has a version of zlib hanging out on the filesystem;
avoid trying to use it as it's either 64 _or_ 32 bit, so exactly one of
our builds will fail.

azure-pipelines.yml

@@ -96,7 +96,7 @@ jobs:
     parameters:
       environmentVariables:
         CMAKE_GENERATOR: Visual Studio 15 2017
-        CMAKE_OPTIONS: -A x64 -DMSVC_CRTDBG=ON -DDEPRECATE_HARD=ON
+        CMAKE_OPTIONS: -A x64 -DMSVC_CRTDBG=ON -DDEPRECATE_HARD=ON -DUSE_BUNDLED_ZLIB=ON
         SKIP_SSH_TESTS: true
         SKIP_NEGOTIATE_TESTS: true
 
@@ -109,7 +109,7 @@ jobs:
     parameters:
       environmentVariables:
         CMAKE_GENERATOR: Visual Studio 15 2017
-        CMAKE_OPTIONS: -A Win32 -DMSVC_CRTDBG=ON -DDEPRECATE_HARD=ON -DUSE_SHA1=HTTPS
+        CMAKE_OPTIONS: -A Win32 -DMSVC_CRTDBG=ON -DDEPRECATE_HARD=ON -DUSE_SHA1=HTTPS -DUSE_BUNDLED_ZLIB=ON
         SKIP_SSH_TESTS: true
         SKIP_NEGOTIATE_TESTS: true