bug#60494: git-reference does not support file:// uri scheme

[mirai]

I'd like to build some specific programs that are git managed but not uploaded 
anywhere as packages in guix.
The rationale for this is that they're either too specific to be worth 
publishing or the programs are still
in their very early stages of development.

Using the file:// scheme in a package object does not work and the only 
workaround is passing the "--with-source=..."
command-line argument to 'guix build' which is rather selective in what it 
accepts, namely it does not work with bare repositories
meaning that a redundant git clone needs to be done beforehand.