Improve version extraction from the GitHub ref for the Stryker dashboard

So that reports are available at https://dashboard.stryker-mutator.io/reports/github.com/serilog-contrib/serilog-formatting-log4net/1.0.0 instead of https://dashboard.stryker-mutator.io/reports/github.com/serilog-contrib/serilog-formatting-log4net/refs/tags/1.0.0 (i.e. without the refs/tags part)

See https://stackoverflow.com/questions/58033366/how-to-get-the-current-branch-within-github-actions/58034787#58034787

.github/workflows/continuous-integration.yml

@@ -76,7 +76,7 @@ jobs:
         run: |
           cd tests
           dotnet tool restore
-          dotnet tool run dotnet-stryker --reporter dashboard --version ${GITHUB_REF#refs/heads/} --dashboard-api-key ${{ secrets.STRYKER_DASHBOARD_API_KEY }}
+          dotnet tool run dotnet-stryker --reporter dashboard --version ${GITHUB_REF##*/} --dashboard-api-key ${{ secrets.STRYKER_DASHBOARD_API_KEY }}
       - name: Create NuGet package
         run: dotnet pack --no-build --output .
         id: dotnet-pack