Dear guix,

nsis is broken from the switch to GCC 10. See here:
https://sourceforge.net/p/nsis/bugs/1248/

The fix here is to simply update the package to a newer version. Doing
this, we can drop the SOURCE_DATE_EPOCH patch, but we must keep the
environment hack for the build system. I just updated it to the new
syntax.

What do you think?

Best regards,

Vivien