build: Disable macos-11

This specific environment seems to be more prone to a data race
that is already fixed on the git-refactoring branch. Whilst those
changes are not merged into main, let's disable this environment
to quieten the unnecessary noise.

Note that this test only exists to ensure that contributors using
macos-11 as their development environment don't experience
issues building and debugging the project.

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

.github/workflows/tests.yaml

@@ -71,7 +71,7 @@ jobs:
   darwin-amd64:
     strategy:
       matrix:
-        os: [macos-11, macos-12]
+        os: [macos-12]
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: