* guix/build/julia-build-system.scm: Remove phases trailing #t.
[phases]{check}: Omit the --procs argument when parallel tests are to be
disabled. Substract 1 from the number of parallel jobs otherwise.
Co-authored-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>