guix: Bump to upstream commit with mingw-w64 changes

Most of the mingw-w64 toolchain changes have now been upstreamed, we can
point to a commit that exists upstream.

NOTE: I'm not changing the URL yet until we see that Guix upstream will
accept all my patches for macOS.

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The Guix tree that's referred to by this commit contains the following
changes relevant to our mingw-w64 build:

b066c25026

  Adds a PACKAGES-WITH-*PATCHES procedure which we can use in the future
  to apply patches to packages if those patches are not considered
  appropriate to upstream Guix

4719b71572

  Adds mingw-w64 (the libc itself) reproducibility patches, taken from
  debian.

79825bee07 + 401d28e433 + c1c50cb5b0

  Add mingw-w64 specific binutils patches, taken from debian.
  Specifically, the "Make DLL import libraries reproducible" patch made
  libbitcoinconsensus.dll.a build reproducibly. The followup commits
  were hotfixes for my mistakes.

0f864175dc

  Bumps mingw-w64 to v7.0.0. This is the first release that enables
  secure APIs by default (which we need), and gains _FORTIFY_SOURCE
  support. This will also be what Ubuntu Focal 20.04 LTS releases with.

cdf00cf75d

  Bumps NSIS to v3.05. This is the first release that includes a fix for
  a reproducibility bug found by some of the electrum developers. See
  details here: https://sourceforge.net/p/nsis/bugs/1230/

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