CI: Set GIT_REF as an environment variable

An environment variable, GIT_REF, has been configured in the test workflow. Its value is defined based on GitHub event repository's default branch. This has been done to standardize the reference used across different steps of the test job. As a result, the run command for mutation.sh has also been simplified and made more readable.

Author: devtekve

.github/workflows/tests.yml

@@ -94,6 +94,8 @@ jobs:
     name: Safety mutation tests
     runs-on: ${{ github.repository == 'commaai/opendbc' && 'namespace-profile-amd64-8x16' || 'ubuntu-latest' }}
     timeout-minutes: 20
+    env:
+      GIT_REF: ${{ github.event_name == 'push' && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && github.event.before || format('origin/{0}', github.event.repository.default_branch) }}
     steps:
       - uses: actions/checkout@v4
         with:
@@ -103,4 +105,4 @@ jobs:
         run: |
           source setup.sh
           scons -j8
-          GIT_REF=$${{ github.event_name == 'push' && (github.ref == 'refs/heads/master' || github.ref == 'refs/heads/master-new') && github.event.before || ('origin/master' || 'origin/master-new') }} cd opendbc/safety/tests && ./mutation.sh
+          cd opendbc/safety/tests && ./mutation.sh