[release/8.0.1xx-xcode16.0] [actions] The changelog action now requir…

…es .NET 9. (#21630)

So install that, since it doesn't look like it's installed by default yet.

Also build before running, because the new terminal logger for the build
in .NET 9 ends up printing junk which ends up in the changelog.

Backport of #21629

---------

Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>

.github/workflows/maestro-changelog.yml

@@ -15,12 +15,17 @@ jobs:
     if: github.actor == 'dotnet-maestro[bot]'
 
     steps:
+    - uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '9'
+
     - name: 'Compute changelog'
       run: |
         set -exo pipefail
         git clone https://github.com/spouliot/dotnet-tools
         cd dotnet-tools/changelog
-        dotnet run https://github.com/$GITHUB_REPOSITORY/pull/${GITHUB_REF_NAME/\/*/} > /tmp/changelog.txt 2>&1
+        dotnet build
+        ./bin/Debug/net9.0/changelog https://github.com/$GITHUB_REPOSITORY/pull/${GITHUB_REF_NAME/\/*/} > /tmp/changelog.txt 2>&1
 
     - name: 'Add changelog'
       uses: actions/github-script@v7.0.1