Docs: User Guide example cache persists through music-suite library source code change

[hanshoglund]

Reproduce:

1. cabal build && (cd docs && make)
2. Change something under src that would affect lilypond output - e.g. replace toLy with undefined.
3. cabal build && (cd docs && make)

Expected:

• All Lilypond examples are regenerated (e.g. failing if toLy is undefined).

Actual:

• Nothing happens as examples are cached.

Workaround:
(cd docs && make clean) before running any test/command. Note that without this ci is not reproducible.