CI: Try to not auto-cancel on master

[lithomas1]

• closes CI: disable auto-cancel for master branch #42076
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

Seems to work. Let's try this again.

[jreback]

@lithomas1 works? or we should just merge and see?

[lithomas1]

Yup, it seems to not cancel on a fake 1.3.x branch that I created locally and seems to cancel correctly on the pull request here properly. I'll still monitor to make sure it's working properly, though.

[jreback]

thanks @lithomas1

[jbrockmendel]

I just got 14 emails about a single CI run being cancelled. This is the most recent CI-centric PR merged AFAICT. Any chance there's a connection?

[jorisvandenbossche]

I also just noticed this. Github actions is complaining about a line that was added in this PR (see eg https://github.com/pandas-dev/pandas/actions/runs/1535966470/workflow), but the strange thing is that this only started failing more recently on master (so after this PR was merged, there have been several other commits where github actions still ran)