workflows: static-checks: Free disk space before running checks

We've been seeing the 'sudo make test' job occasionally run out of space in
/tmp, which is part of the root filesystem. Removing dotnet and
`AGENT_TOOLSDIRECTORY` frees around 10GB of space and in my tests the job still
has 13GB of space left after running.

Fixes: #6401
Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

.github/workflows/static-checks.yaml

@@ -23,6 +23,10 @@ jobs:
       target_branch: ${{ github.base_ref }}
       GOPATH: ${{ github.workspace }}
     steps:
+    - name: Free disk space
+      run: |
+        sudo rm -rf /usr/share/dotnet
+        sudo rm -rf "$AGENT_TOOLSDIRECTORY"
     - name: Checkout code
       uses: actions/checkout@v3
       with: