Allow build mount path to be a parent of $GITHUB_WORKSPACE

This release adds support for mounting the build volume over a parent folder of $GITHUB_WORKSPACE.

Previously, doing so would remove/shadow the current working directory, and make any later actions/steps fail.

New features:

• Warn about $BUILD_MOUNT_PATH not being empty and list contents in that case
• Recreate $GITHUB_WORKSPACE afterwards if missing, so following actions have a working directory to start from.