Automate packaging built copies of CakeML

HOL supports packaging a built development for reuse on another machine via its `relocbuild` argument to `build` and `Holmake`. However, the packaging and reuse process is a bit fiddly and complicated. Some notes on how to do it are at https://github.com/CakeML/regression/blob/master/make-relocatable-tarball.sh. This issue is to automate the process with either an executable (written in CakeML) or a `Holmake` target. In particular, the result should support
- Downloading a built copy of the entire CakeML repo (and its corresponding HOL repo) for reuse locally
- Development (e.g., hot-fixing) on such a relocated copy, so that results can be pushed back to GitHub
- Optionally: scalability to supporting additional (CakeML-dependent) repos

The ideal workflow would be something like:
1. Download `cakeml-XYZ.tar.gz` and unpack it
2. Run `` inside `cakeml-XYZ`
3. Now `cakeml-XYZ/{HOL,cakeml}` contains usable built repos on which to do further development

And on the packaging side:
1. cd `cakeml-XYZ` - a built (or semi-built) working copy of the CakeML repo
2. run `` to produce `cakeml-XYZ.tar.gz` for use as above