Move git version env var setting to a warbo-utilities script

[Chris Warburton]

We use latestGit a lot in Nix derivations, to check what the current
HEAD of a git repo is. This can take a couple of seconds during
evaluation time, which can add up if we're checking 50+ repos.

There's a mechanism built in which will check for an env var, matching
the hash of the repo's name; if found, that env var's value will be used
instead.

We use this in ./render to great effect, but it would be nice to use it
elsewhere too, e.g. in general tests.

Pull the code out of ./render and move it to a script in
warbo-utilities.