GHA/checksrc: add actionlint, fix or silence issues found

GHA/checksrc: add actionlint, fix or silence issues found

It also does shellcheck on `run:` elements, overlapping with
the homegrown `shellcheck-ci.sh` with the same purpose. But it also does
more and perhaps could replace the script too, especially in curl
sub-repos.

Also:
- GHA/macos: delete potentially useful, but commented, and ultimately
  unused, non-default Xcode-testing logic. It's causing unused matrix
  exceptions, upsetting actionlint.

Ref: https://github.com/rhysd/actionlint

Closes #19373