bug#36925: guix pack -R or -RR will not produce outputs other than 'out'

[Jesse Gibbons]

This happens with all the packages I have tried. For example,
`guix pack --dry-run -R ghc:doc` fails with the message:

$ guix pack --dry-run -R ghc:doc
guix pack: error: reference to invalid output 'doc' of derivation
'/gnu/store/a0blj0d79mzgvb4fk8fkai51xjf68s7z-ghc-8.4.3R.drv'

I get similar results if I change -R to -RR:

$ guix pack --dry-run -RR ghc:doc
guix pack: error: reference to invalid output 'doc' of derivation
'/gnu/store/905g4g323i8ngdfnmqdgdm89z66ikj07-ghc-8.4.3R.drv'

However, if I leave out the -R or -RR flag, it succeeds:

$ guix pack --dry-run ghc:doc
substitute: updating substitutes from 'https://ci.guix.gnu.org'...
100.0%
The following derivations would be built:
   /gnu/store/4qnslab7r748r44ydc3zcni0zgfsicak-tarball-pack.tar.gz.drv
   /gnu/store/p3k0xh40ypv7vnvd70jarswn0yykvi8m-profile.drv
0.0 MB would be downloaded:
   /gnu/store/sw8vw31fnmfrjhb32sxvvy0yxrx9s1hc-mkfontscale-1.2.1
   /gnu/store/3rgvdjy72vqsv45f85b0phpirnzxb4w2-mkfontdir-1.0.7
The following profile hooks would be built:
   /gnu/store/1rlk94msgnanr5yyhmx5hck5b3xvsmxa-info-dir.drv
   /gnu/store/56qn66jcnwg8rkz7w0mfw0qkx1fx997n-ghc-package-cache.drv
   /gnu/store/8bpnr5pkzjnlj1zzrcjgr26x1yyyw82l-manual-database.drv
   /gnu/store/d7l70fw7zyg7mpkxnm87v69cphfi4s9z-fonts-dir.drv
   /gnu/store/f4xcbmbzzq32g8ywy8z96pyr89qni1g6-ca-certificate-
bundle.drv

The --dry-run flag is not necessary to replicate this bug. I included
it to keep success output short and (hopefully) help narrow down when
the bug happens.

-- 
-Jesse