Additional action steps are unable to access the Magento build that is local to the /tmp/m2 directory.

Proposed solution:

1. Check out the module source to a subdirectory of GITHUB_WORKSPACE, e.g. $GITHUB_WORKSPACE/source.
2. Change the build directory to $GITHUB_WORKSPACE/build.

This already seems to have been intended at one point of time according to the otherwise unused $MODULE_SOURCE variable when the local-source is set up.

This would allow adding additional steps that can reference the existing build.