Pin Legion to specific commit sha by default

[trxcllnt]

This PR pins the Legion repo URL and git revision that's built if Legion is not found via find_package().

It explicitly ignores values stored in the CMakeCache.txt, so subsequent rebuilds that don't set the --legion-{url,branch} args revert to the defaults:

$ install.py --editable --legion-branch "feature_branch"

# Reverts to building the default pinned Legion sha, not "feature_branch"
$ install.py --editable

[marcinz]

Is it possible to move the SHA for Legion to versions.json? I think we discussed it at some point, but I wonder how hard it is.

[magnatelee]

Is it possible to move the SHA for Legion to versions.json?

+1

[trxcllnt]

Yes, that's possible.