Tell GitHub not to cancel builds in the posix matrix

[rptb1]

Fixes #295 according to Handling failures.

[rptb1]

Executing proc.review.express.

1. Start time 11:43.
2. Applying entry.universal. Note that for entry.universal.auto-check all automatic checks have not passed (macos-latest build failed) but that is expected, and useful, since it demonstrated that the other posix matrix builds were not cancelled.
3. Calling in @thejayps . There will be a short delay.

[thejayps]

4. express.risk, agree low risk and appropriate for express review
5. Source docs: Running variations of jobs in a workflow
6. No defects found by me or @rptb1
7. .express.brainstorm @rptb1 "could we have noticed that this could be an issue when setting up the build? Can't think of any process improvement to make"
8. Executing proc.review.exit
9. Exit Start time 12:22
10. Change passed.
11. review.exit.calc:

• hours used: 0.5
• hours saved: ~5-6h repeating builds
• major defects remaining: 0

12. Exit finished at 12:28: 6 mins

[thejayps]

exit.pass

[rptb1]

Executing proc.merge.pull-request

1. Start time 12:27.
2. All checklist items pass except "pass tests" as noted above. The "automated test case" is to notice cancelled builds.
3. Using GitHub's "Merge button" to replace section 5, "Merging procedure", because this is a one-commit PR that does not require a local build.
4. End time 12:31.