PR checks: stop running unset-env on two old CLI versions

Looks like there was a bug in the oldest two CLI versions, on `macos-12` here.

Author: angelapwen

.github/workflows/__unset-environment.yml

@@ -1,5 +1,7 @@
 name: "Test unsetting environment variables"
 description: "An end-to-end integration test that unsets some environment variables"
+# TODO: Switch back to all versions once CLI v2.17.4 is available and running on ubuntu again.
+versions: ["stable-v2.14.6", "stable-v2.15.5", "stable-v2.16.6", "latest", "default", "nightly-latest"] 
 operatingSystems: ["macos"] # TODO: Switch back to ubuntu for `nightly-latest` and `latest` once CLI v2.17.4 is available.
 steps:
   - uses: ./../action/init