ci: make the "skip-if-redundant" check more defensive

ci: make the "skip-if-redundant" check more defensive

In 7d78d5fc1a9 (ci: skip GitHub workflow runs for already-tested
commits/trees, 2020-10-08), we added a check that determines whether
there is already a workflow run for the given commit (or at least tree),
and if found, skips the current run.

We just worked around an issue with this check where older runs might
unexpectedly miss the `head_commit` attribute.

Let's be even more defensive by catching all kinds of exceptions,
logging them as warnings, and continue the run without skipping it
(after all, if the check fails, we _want_ to continue with the run).

This commit is best viewed with the diff option `-w` because it
increases the indentation level of the GitHub Action script by two
spaces, surrounding it by a `try ... catch` construct.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>