bug#31085: Unexpected behaviour when running `guix build lilypond'

[Diego Nicola Barbato]

Hello Guix

I have experienced some unexpected behaviour when running `guix build
lilypond':
After verifying that there was a substitute available with `guix weather
-m m.scm' (Where m.scm evaluates to a manifest containing only lilypond)
I ran `guix build lilypond --dry-run' which claimed that a substitute
would be downloaded.  I then ran `guix build lilypond' which proceeded
to build lilypond from source (instead of downloading the substitute).

Upon explaining this on IRC it was suggested that I try running `guix
build --no-grafts lilypond' (which actually downloaded the substitute)
then deleting the locally built lilypond with `guix gc --delete
/gnu/store/...' and finally running `guix build lilypond' again (which,
this time, grafted the substituted lilypond instead of building it from
source again).  While this fixed the issue I was told that this
behaviour was indeed unexpected.

I run GuixSD (commit: ae81bf4f995466a4650e2756d2763b8a163d2f63) on an
x86-64 machine.

Greetings

Diego