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

.github/workflows/http3-linux.yml

@@ -666,8 +666,6 @@ jobs:
 
       - name: 'run tests'
         if: ${{ !contains(matrix.build.install_steps, 'skipall') && !contains(matrix.build.install_steps, 'skiprun') }}
-        env:
-          TFLAGS: '${{ matrix.build.tflags }}'
         run: |
           source ~/venv/bin/activate
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then