ENH: GHA: cancel in progress builds for pull requests

[SimonRit]

Based on InsightSoftwareConsortium/ITK#5019.

I based it on v5.4.0, let me know if this is adequate.

[dzenanz]

Have you tried this in any remote modules?

[SimonRit]

I just tried it:

• created ENH: GHA: cancel in progress builds for pull requests RTKConsortium/RTK#650
• amended (randomly) one file
• checked that the previous actions were cancelled (with the message "Canceling since a higher priority waiting request for 'Build, test, package@concurrency' exists")

Looks good to me!

[dzenanz]

#99 proposes this for main branch.