ci: west_cmds: Use specific version of runner image

This commit updates the "West Command Tests" workflow to use a specific
runner image version (ubuntu-20.04, macos-11, windows-2022) instead of
the latest version in order to prevent any potential breakages due to
the 'latest' version change by GitHub.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>