svn-download: Reduce svn-fetch 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 svn-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
0e73f933b291c7e154c7e019b6de1e2f3a97e4c1.  I noticed the hash in the builder
script when wondering why the build coordinator on bayfront was substituting
svn-multi-download files over and over again.

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

Change-Id: I256b94666296ad747f494f0b497ca209b77fbfb4

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