feat: add test for EXTENSIONS_GALLERY (#5432)

* refactor: add env arg to runCodeServerCommand

This allows yous to pass environment variables to code-server's helper
when running integration tests.

* feat: add EXTENSIONS_GALLERY integration test

This test ensures EXTENSIONS_GALLERY is read when set and using the
`--install-extension` flag.

Co-authored-by: repo-ranger[bot] <39074581+repo-ranger[bot]@users.noreply.github.com>