git-download: Reduce builder duplication.

Rather than creating a different builder in the store for every different
download (by hash), remove the hash from the builder and pass it in via an
environment variable.  This means that when git-fetch is used by two different
package sources, the derivations will still differ but the builder will be
shared.

It used to be this way, but changed with
264fdbcaff9c078642355bace0c61c094b3581fc.  I noticed this through looking at
the same problem with svn-multi-fetch.

To try and make the effects of introducing variance in to the builder script
more obvious, separate it out in to it's own procedure, so that it's clearer
when there's new data going in that could cause variance.

* guix/git-download.scm (git-fetch/in-band*): Extract out builder script,
include hash in the derivation as an environment variable and update the
comment to be more directive.
(git-fetch-builder): New procedure.

Change-Id: I59c9fc445667c0e7dc44bcb706818300c394a1e5

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