static-test: use base reference as base branch instead of master

This way they will also work with release branches ;-).

.github/workflows/static-test.yml

@@ -21,15 +21,19 @@ jobs:
         fetch-depth: 0
     - name: Setup git
       run: |
-        git fetch origin master:master_upstream --no-tags
+        # Note: ${{ github.base_ref }} is empty when not in a PR
+        if [ -n "${{ github.base_ref }}" ]; then
+          git fetch origin ${{ github.base_ref }}:${{ github.base_ref }} --no-tags
+        fi
         git config apply.whitespace nowarn
     - name: Fetch riot/riotbuild Docker image
       run: docker pull riot/riotbuild:latest
     - name: Run static-tests
       run: |
-        docker run --rm                     \
-          -e CI_BASE_BRANCH=master_upstream \
-          -e GITHUB_RUN_ID=${GITHUB_RUN_ID} \
-          -v $(pwd):/data/riotbuild         \
-          riot/riotbuild:latest             \
+        # Note: ${{ github.base_ref }} is empty when not in a PR
+        docker run --rm                             \
+          -e CI_BASE_BRANCH=${{ github.base_ref }}  \
+          -e GITHUB_RUN_ID=${GITHUB_RUN_ID}         \
+          -v $(pwd):/data/riotbuild                 \
+          riot/riotbuild:latest                     \
           make static-test