GHA: reintroduce an env var for `$GITHUB_HEAD_REF` (#10659)

This should ensure GHA runs synapse against the same-named sytest branch

.github/workflows/tests.yml

@@ -192,6 +192,7 @@ jobs:
       volumes:
         - ${{ github.workspace }}:/src
       env:
+        SYTEST_BRANCH: ${{ github.head_ref }}
         POSTGRES: ${{ matrix.postgres && 1}}
         MULTI_POSTGRES: ${{ (matrix.postgres == 'multi-postgres') && 1}}
         WORKERS: ${{ matrix.workers && 1 }}

changelog.d/10659.misc

+Fix GitHub Actions config so we can run sytest on synapse from parallel branches.
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