#5328 mf2pt1 uses random uniqueID

[Bernhard M. Wiedemann]

scripts/build/mf2pt1.pl calls int(rand(1000000)) + 4000000
so with every (clean) build, a new uniqueID value gets generated, which prevents lilypond packages from building reproducibly.
See also https://reproducible-builds.org/ for why this matters.

One pretty clean solution would be to calculate a hash of all relevant inputs and use that for uniqueID.

Since I dont know what the inputs are there, for now I only did a dummy patch that hashed the filename...