fix(review): Remove --source argument to match CLI 8.0 release (#347)

Gemini CLI 8.0 release introduced
[PR#10628](google-gemini/gemini-cli#10628) which
removes the need for a path/source argument when installing extensions
and breaks run-gemini-cli's extension installation.

Removing `--source` and relying on positional arguments patches
`run-gemini-cli` for Gemini CLI 8.0+

I've tested this locally as well in
CallumHYoung/testrepo#3 which relies on
https://github.com/CallumHYoung/run-gemini-cli

Author: CallumHYoung

action.yml

@@ -230,7 +230,7 @@ runs:
             extension=$(echo "${extension}" | xargs)
             if [[ -n "${extension}" ]]; then
               echo "Installing ${extension}..."
-              echo "Y" | gemini extensions install --source "${extension}"
+              echo "Y" | gemini extensions install "${extension}"
             fi
           done
         fi