The environment variable SOURCE_DATE_EPOCH allows overriding the date that will be used inside the archives for guix-built binaries. This is an intentional feature, as documented in contrib/guix/README.md:

• SOURCE_DATE_EPOCH

  Override the reference UNIX timestamp used for bit-for-bit reproducibility,
  the variable name conforms to [standard][r12e/source-date-epoch].

  (defaults to the output of $(git log --format=%at -1))

However, some environments, as apparently Nix, set SOURCE_DATE_EPOCH by default. This can be extremely confusing, as it results in mismatches. i see three options:

• Show a big red warning when the Guix build scripts are called with an existing SOURCE_DATE_EPOCH. Most likely it's not what the user wanted, but they can continue if they wanted.
• Disallow overriding SOURCE_DATE_EPOCH - unset it at start.
• Rename our SOURCE_DATE_EPOCH to something non-standard that doesn't conflict with Nix.

The first one has my preference.

See bitcoin-core/guix.sigs#1201 which prompted this.