This allows 'git-fetch' etc. origins to honor the proxy and locale of
the daemon.
* guix/bzr-download.scm (bzr-fetch): Pass #:leaked-env-vars to
'gexp->derivation'.
* guix/cvs-download.scm (cvs-fetch): Likewise.
* guix/git-download.scm (git-fetch): Likewise.
* guix/hg-download.scm (hg-fetch): Likewise.
* guix/svn-download.scm (svn-multi-fetch): Likewise.