ci(win+VS): download the vcpkg artifacts using a dedicated GitHub Action

The Git for Windows project provides a GitHub Action to download and
cache Azure Pipelines artifacts (such as the `vcpkg` artifacts), hiding
gnarly internals, and also providing some robustness against network
glitches. Let's use it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -181,14 +181,10 @@ jobs:
         repository: 'microsoft/vcpkg'
         path: 'compat/vcbuild/vcpkg'
     - name: download vcpkg artifacts
-      shell: powershell
-      run: |
-        $urlbase = "https://dev.azure.com/git/git/_apis/build/builds"
-        $id = ((Invoke-WebRequest -UseBasicParsing "${urlbase}?definitions=9&statusFilter=completed&resultFilter=succeeded&`$top=1").content | ConvertFrom-JSON).value[0].id
-        $downloadUrl = ((Invoke-WebRequest -UseBasicParsing "${urlbase}/$id/artifacts").content | ConvertFrom-JSON).value[0].resource.downloadUrl
-        (New-Object Net.WebClient).DownloadFile($downloadUrl, "compat.zip")
-        Expand-Archive compat.zip -DestinationPath . -Force
-        Remove-Item compat.zip
+      uses: git-for-windows/get-azure-pipelines-artifact@v0
+      with:
+        repository: git/git
+        definitionId: 9
     - name: add msbuild to PATH
       uses: microsoft/setup-msbuild@v2
     - name: copy dlls to root