Consider adding linting for Github action ymls

[mkannwischer]

https://github.com/rhysd/actionlint finds various problems in our Github actions:

1. base.yml:94 — Duplicate matrix entry for aarch64/ubuntu-24.04-arm (line 92 and 94 are identical)
2. bench.yml:98 — matrix.opt.name references undefined property opt; the matrix only defines target
3. bench_ec2_reusable.yml:179,195 — Input custom-shell doesn't exist on the bench action; the actual
   input is custom_shell (underscore not hyphen). Silently ignored, so the custom shell is never set.
4. ci.yml:585 — matrix.target.ec2_ami_id references undefined property; the matrix defines ec2_ami
   not ec2_ami_id
5. integration-awslc.yml:167 — ${{ matrix.flags }} should be ${{ matrix.test.flags }} in the prefix
   job (this one actionlint didn't flag explicitly, but we found it manually earlier)

We should run that in CI using https://github.com/raven-actions/actionlint