bug#25449: GHC packages take a long time to install

[David Craven <david@craven.ch>]

Hi Ludo!

guix build idris -n works for me too.

guix build idris --no-substitutes does this (which may be a problem on my end):
guix build idris --no-substitutes
warning: failed to install locale: Invalid argument
@ build-started
/gnu/store/85rrbgf0y77s1qwk9lw04gcf44nhw213-libxslt-1.1.29.tar.gz.drv
- x86_64-linux /var/log/guix/drvs/85//rrbgf0y77s1qwk9lw04gcf44nhw213-libxslt-1.1.29.tar.gz.drv.bz2
error: failed to run download program
'/home/dvc/guix/nix/scripts/download': Permission denied
builder for `/gnu/store/85rrbgf0y77s1qwk9lw04gcf44nhw213-libxslt-1.1.29.tar.gz.drv'
failed with exit code 1
@ build-failed /gnu/store/85rrbgf0y77s1qwk9lw04gcf44nhw213-libxslt-1.1.29.tar.gz.drv
- 1 builder for
`/gnu/store/85rrbgf0y77s1qwk9lw04gcf44nhw213-libxslt-1.1.29.tar.gz.drv'
failed with exit code 1
@ build-started
/gnu/store/69ln3nw2m9ydkjk5c6z72rpjlsdypl0x-Python-3.5.2.tar.xz.drv -
x86_64-linux /var/log/guix/drvs/69//ln3nw2m9ydkjk5c6z72rpjlsdypl0x-Python-3.5.2.tar.xz.drv.bz2
cannot build derivation
`/gnu/store/ddn0hzpn8nc4rdy8hwns2hh49761wgqc-libxslt-1.1.29.tar.xz.drv':
1 dependencies couldn't be built
cannot build derivation
`/gnu/store/y8q83qq1ggvm0z9p0g3a8lfzbghgcrfl-libxslt-1.1.29.drv': 1
dependencies couldn't be built
guix build: error: build failed: build of
`/gnu/store/y8q83qq1ggvm0z9p0g3a8lfzbghgcrfl-libxslt-1.1.29.drv'
failed

When I do git checkout 3254e1aba9ce3925a4e0b24c647240f96b5de8d4 (the
last commit of the series that added idris)
guix build idris works fine too. This is what lead me to believe that
the problem was not on my end.

Thanks!
David