When building nsis, if VERSION is not specified, it defaults to
cvs_version which is non-deterministic as it includes the current date.
This patches nsis to default to SOURCE_DATE_EPOCH if it exists so that
nsis is reproducible.
Upstream change: https://github.com/kichik/nsis/pull/13
Sidenote: also a good demonstration of how Guix allows us to flexibly patch our tools!
Note to reviewers: if you want to compare hashes, please build after Jan 16th 2021 without my substitute server enabled!