bug#42420: git-fetch origins produce same store output when set recursive is set to true or false

[pkill9]

I built a source that uses git-fetch - by default (recursive? #f) - and
the package failed to build, then I remembered it uses submodules, so I
set (recursive? #t) but it failed with the same error. The problem is
that the store path of the source is the same for both, so it didn't
try to rebuild the git checkout with submodules checked out.

After garbage collecting the source so it rebuilds it, I can build the
package.