mediagoblin guix package some outputs are not valid, so checking is not possible

[jgart]

Hi when running `guix build -L . mediagoblin --check`

I get the following error:

```
stripping binaries in 
"/gnu/store/8gdlglv98ilnhjqg25ag63m1n8kcpjkd-mediagoblin-0.12.0.dev.1/bin" with 
"strip" and flags ("--strip-debug" "--enable-deterministic-archives")
phase `strip' succeeded after 0.2 seconds
starting phase `validate-runpath'
validating RUNPATH of 0 binaries in 
"/gnu/store/8gdlglv98ilnhjqg25ag63m1n8kcpjkd-mediagoblin-0.12.0.dev.1/lib"...
validating RUNPATH of 0 binaries in 
"/gnu/store/8gdlglv98ilnhjqg25ag63m1n8kcpjkd-mediagoblin-0.12.0.dev.1/bin"...
phase `validate-runpath' succeeded after 0.1 seconds
starting phase `validate-documentation-location'
phase `validate-documentation-location' succeeded after 0.0 seconds
starting phase `delete-info-dir-file'
phase `delete-info-dir-file' succeeded after 0.0 seconds
starting phase `patch-dot-desktop-files'
phase `patch-dot-desktop-files' succeeded after 0.0 seconds
starting phase `install-license-files'
installing 2 license files from '.'
`./licenses/CC0_1.0.txt' -> 
`/gnu/store/8gdlglv98ilnhjqg25ag63m1n8kcpjkd-mediagoblin-0.12.0.dev.1/share/doc/mediagoblin-0.12.0.dev.1/CC0_1.0.txt'
`./licenses/LGPLv3.txt' -> 
`/gnu/store/8gdlglv98ilnhjqg25ag63m1n8kcpjkd-mediagoblin-0.12.0.dev.1/share/doc/mediagoblin-0.12.0.dev.1/LGPLv3.txt'
`./licenses/AGPLv3.txt' -> 
`/gnu/store/8gdlglv98ilnhjqg25ag63m1n8kcpjkd-mediagoblin-0.12.0.dev.1/share/doc/mediagoblin-0.12.0.dev.1/AGPLv3.txt'
phase `install-license-files' succeeded after 0.0 seconds
starting phase `reset-gzip-timestamps'
phase `reset-gzip-timestamps' succeeded after 0.0 seconds
starting phase `compress-documentation'
phase `compress-documentation' succeeded after 0.0 seconds
successfully built 
/gnu/store/g3qf6fskw5mcw6bvvcv3bfk8ibgmn1mq-mediagoblin-0.12.0.dev.1.drv
The following graft will be made:
   /gnu/store/ih60pb601m6xhdhaa3z889ygbq8z6qr3-mediagoblin-0.12.0.dev.1.drv
guix build: error: some outputs of 
`/gnu/store/ih60pb601m6xhdhaa3z889ygbq8z6qr3-mediagoblin-0.12.0.dev.1.drv' are 
not valid, so checking is not possible
```

Just wanted to share that here in case it is helpful later on in debugging 
related to the package definition.

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