ci: add conditional skip for docs changes

[zalimeni]

Similar to consul, skip build and tests if we're only changing docs.

This should help speed up releases and daily work quick-fixes.

❯ SKIP_CHECK_BRANCH=zalimeni/conditional-skip-ci GITHUB_OUTPUT="../GITHUB_OUTPUT" .github/scripts/check_skip_ci.sh
checking file: .changelog/100.txt
checking file: .github/scripts/check_skip_ci.sh
checking file: README.md
Changes detected in only documentation files - skipping tests and build

After adding a code change:

❯ SKIP_CHECK_BRANCH=zalimeni/conditional-skip-ci GITHUB_OUTPUT="../GITHUB_OUTPUT" .github/scripts/check_skip_ci.sh
checking file: .changelog/100.txt
checking file: .github/scripts/check_skip_ci.sh
checking file: README.md
checking file: cmd/consul-dataplane/main.go
non-skippable file changed: cmd/consul-dataplane/main.go
Changes detected in non-documentation files - will not skip tests and build

[zalimeni]

We were not converting to an array in the original script - it happened to still work due to multiline matches, but this allows us to print individual debug logs more accurately

[zalimeni]

See check here: https://github.com/hashicorp/consul-dataplane/actions/runs/9504971128/job/26198781967#step:3:9

[zalimeni]

@nathancoleman or @DanStough when you get a sec, could I get a follow-up review of this change too? (basically the same as #560 apart from exempt directory list and integrated workflows)

[DanStough]

So the action won't get stuck because this job is required and there are cases where we won't run it, correct?

[zalimeni]

Exactly. This follows the pattern in consul here

[zalimeni]

Removed dead env var accidentally left in from consul script

[DanStough]

Thank you!