github: provide base ref for preprocess check

For preprocess checks on pull requests, check against the base_ref of
the pull request (e.g. master) instead of against the seL4 revision in
the verification manifest.

If base_ref has advanced over the verification manifest revision, we do
not want to see that difference again on pull requests, but only the
difference the pull request causes itself.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

.github/workflows/pr.yml

@@ -33,3 +33,4 @@ jobs:
       with:
         L4V_ARCH: ${{ matrix.arch }}
         L4V_FEATURES: ${{ matrix.feature }}
+        BASE_REF: ${{ github.base_ref }}