--frozen-lockfile is deprecated in yarn3

[w0ofy]

The action uses the --frozen-lockfile flag for the yarn install step, but that flag is deprecated in yarn@v3.2.1 — the yarn resolution step is failing when running the install steps on the merged commits (e.g. PR's commit merged with main).

Here is an example of the failure described above: https://github.com/bigwoof91/real-system/runs/8051667671?check_suite_focus=true

Could there be a way to optionally use the the new CLI flag, --immutable ?