describe: 'current-channel-entries' ignores non-channel profile entries.
Fixes <https://bugs.gnu.org/48778>.
A side effect of
c47f3fc13562d82edfd2d47342574154c452843a is that
(@ (guix describe) current-profile) provides the correct answer when the
'guix' package is installed in a profile. Consequently, the 'guix'
package installed in /run/current-system/profile (for instance) would
end up loading all the .scm files in that directory.
* guix/describe.scm (current-channel-entries): Remove ENTRY if it lacks
the 'source' property.