Add an if condition to invoke Snyk only on Linux.

.github/workflows/pr.yml

@@ -24,6 +24,7 @@ jobs:
         with:
           codecov_token: ${{ secrets.CODECOV_TOKEN }}
       - name: Run Snyk to check for vulnerabilities
+        if: runner.os == 'Linux'
         uses: snyk/actions/dotnet@master
         env:
           SNYK_TOKEN: ${{ secrets.SNYK_TOKEN }}