[guix] Make the OSX DMG generation with xorrisofs deterministic

Summary:
If the SOURCE_DATE_EPOCH environment variable is set, use it.

Partial backport of core#17920:
https://github.com/bitcoin/bitcoin/pull/17920/commits/f3835dc6a3732dcd4afbb5987f84dc27f2bf55af

Test Plan:
Run twice:

HOSTS="x86_64-apple-darwin" ./contrib/guix/guix-build

Check the hashes are identical

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15645