Backport of ci: fix CI skip script hole into release/1.1.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #4323 to be assessed for backporting due to the inclusion of the label backport/1.1.x.

The below text is copied from the body of the original PR.

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In some environments, the script will not fail despite SKIP_CHECK_BRANCH being unset, leading to the script explicitly skipping CI when it should fail fast.

Prevent this by explicitly checking for the env var.

Also fix a small bug where we're incorrectly requiring ',' in the .changelog directory name, which means we'll never skip changelog entries.

This change is a port of hashicorp/consul#21741.

Testing

❯ ./.github/scripts/check_skip_ci.sh
./.github/scripts/check_skip_ci.sh: line 27: SKIP_CHECK_BRANCH: SKIP_CHECK_BRANCH is required

❯ export SKIP_CHECK_BRANCH=net-10843-cannot-add-tolerations-to-apigateway
❯ ./.github/scripts/check_skip_ci.sh
checking file: .changelog/4313.txt
checking file: .github/scripts/check_skip_ci.sh
checking file: .go-version
non-skippable file changed: .go-version
Changes detected in non-documentation files - will not skip tests and build

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Overview of commits

• 5b04560