Extra env variable present on build

[truelecter]

When using https://github.com/DeterminateSystems/update-flake-lock action to create a new branch and then PR immediately, something strange happens - CirrusCI triggers both builds, which I assume makes sense, but build for the branch itself has the same environment variables (including CIRRUS_PR). The only difference (except small diff in commit/body text-related variables) is the presence of GITHUB_CHECK_SUITE_ID variable on the branch build.

As such, attempting to skip duplicate builds with, for example, only_if: CIRRUS_PR == '' makes both builds to be skipped entirely, which is unexpected at all.

Example of this builds combo:
https://cirrus-ci.com/build/5839811643179008
https://cirrus-ci.com/build/6343569565286400

I've checked environment variables with Debugging information section of appropriate builds