Updated appveyor job to checkout a specific vcpkg commit ID.

Github-Pull: #18001
Rebased-From: 2bcc70531a

.appveyor.yml

@@ -11,11 +11,12 @@ environment:
   QT_DOWNLOAD_HASH: '9a8c6eb20967873785057fdcd329a657c7f922b0af08c5fde105cc597dd37e21'
   QT_LOCAL_PATH: 'C:\Qt5.9.8_x64_static_vs2019'
   VCPKG_INSTALL_PATH: 'C:\tools\vcpkg\installed'
+  VCPKG_COMMIT_ID: 'ed0df8ecc4ed7e755ea03e18aaf285fd9b4b4a74'
 install:
 # Disable zmq test for now since python zmq library on Windows would cause Access violation sometimes.
 # - cmd: pip install zmq
 # Powershell block below is to install the c++ dependencies via vcpkg. The pseudo code is:
-#    a. Update the vcpkg source (including port files) and build the vcpkg binary,
+#    a. Checkout the vcpkg source (including port files) for the specific checkout and build the vcpkg binary,
 #    b. Install the missing packages.
 - ps: |
       $env:PACKAGES = Get-Content -Path build_msvc\vcpkg-packages.txt
@@ -23,6 +24,7 @@ install:
       cd c:\tools\vcpkg
       $env:GIT_REDIRECT_STDERR = '2>&1' # git is writing non-errors to STDERR when doing git pull. Send to STDOUT instead.
       git pull origin master > $null
+      git checkout $env:VCPKG_COMMIT_ID
       git -c advice.detachedHead=false checkout $env:VCPKG_COMMIT_ID
       .\bootstrap-vcpkg.bat > $null
       Add-Content "C:\tools\vcpkg\triplets\$env:PLATFORM-windows-static.cmake" "set(VCPKG_BUILD_TYPE release)"