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GH-35752: [CI][GLib][Ruby] Pass GITHUB_ACTIONS environment variable to Docker containers (#35753)

### Rationale for this change

It's for choosing GitHub Actions suitable output automatically.

test-unit gem checks `GITHUB_ACTIONS=true` and the environment variable is set by GitHub Actions automatically.

### What changes are included in this PR?

Add `GITHUB_ACTIONS:` to `environment:`.

`c_glib/README.md` is updated to run CI jobs.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #35752

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>