ci: allow specifying pmbootstrap path to aid running locally
I don't like how CI requires pmbootstrap in PATH to be a symlink, I use a wrapper script to handle running pmbootstrap how I want and the current pmbootstrap path detection is kinda annoying since it depends on it being a symlink.
This allows using an environment variable, PMBOOTSTRAP_PATH, to set the pmbootstrap path and skip all of the symlink resolving stuff.