github: Prevent Actions running twice in PRs

This is seems to be a common[1] issue[2] and there really is not a very
satisfactory solution other than adding an "if" on the job rather than
on the top-level workflow. Of course, there were other solutions listed,
but they'd involve managing a hardcoded list of branch names where the
"on push" action is triggered.

So I went for the workaround listed in [1], which adds an if to the job
specification.

[1]: zopefoundation/meta#145
[2]: open-telemetry/opentelemetry-python#1370

Signed-off-by: aszlig <aszlig@nix.build>

Author: aszlig

.github/workflows/main.yml

@@ -4,6 +4,9 @@ on: [push, pull_request]
 jobs:
   linux:
     runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request' ||
+      github.event.pull_request.head.repo.full_name !=
+      github.event.pull_request.base.repo.full_name
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v1