hg-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 hg-fetch is used by two different
package sources, the derivations will still differ but the builder will be
shared.

Looking at the code, becuase the ref is also in the builder, the builders have
been duplicated for a while.  The overhead is probably limited though since
hg-reference isn't used much compared to say svn-multi-reference.

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/hg-download.scm (hg-fetch): Extract out builder script and include
hash, hg ref url, and hg ref changeset in the derivation as an environment
variables.
(hg-fetch-builder): New procedure.

Change-Id: I3c3a0b4963ea1b208bf1d5137ef98666458ae2d7

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