With the move from cirrus-ci to GHA, the CIRRUS_REPO_FULL_NAME env var
was always unset, never triggering the sanity check.
Fix this by introducing a new vendor-agnostic env var and setting it
properly.
The CIRRUS_PR env var was cirrus-specific and using a provider-agnostic
name makes more sense.
Also, enable pipefail, while touching this file.
This refactor is needed for the next commit.
Moving the python code out of the yaml string makes it easier to lint,
format, and edit.
This can be reviewed with the git options:
--color-moved=dimmed-zebra --color-moved-ws=ignore-all-space
