build(ci): only run builds supported by the .NET 5 runtime

A separate workflow will target .NET Framework 4.6.1 on a windows host

.github/workflows/build.yaml

@@ -19,15 +19,18 @@ jobs:
         fetch-depth: 0
 
     - uses: actions/setup-dotnet@v3
+      id: dotnetstep
       name: .NET Setup
       with:
-        dotnet-version: 7.0.x
+        dotnet-version: 5.0.x
+
+    - name: Create Temporary Global.json
+      run: echo '{"sdk":{"version":"${{ steps.dotnetstep.outputs.dotnet-version }}", "rollForward":"major"}}' > ./global.json
 
     - name: Install GitVersion
       uses: gittools/actions/gitversion/setup@v0
       with:
-        versionSpec: '6.x'
-        includePrerelease: true
+        versionSpec: '5.x'
 
     - name: Determine Version
       uses: gittools/actions/gitversion/execute@v0