Merge bitcoin/bitcoin#22531: guix: Fixes to guix-{attest,verify}

9b313dfef1 guix: Ensure EPOCH_SOURCE_DATE does not include GPG information (Andrew Chow)
43225f0a2a guix: Remove extra \r from all.SHA256SUMS line ending (Andrew Chow)
d080c27066 guix, doc: Add a note that codesigners need to rebuild after tagging (Andrew Chow)
4a466388a0 guix: Allow changing the base manifest in guix-verify (Andrew Chow)
33455c7696 guix: Make all.SHA256SUMS rather than codesigned.SHA256SUMS (Andrew Chow)

Pull request description:

  `guix-verify` expects `all.SHA256SUMS` but `guix-attest` produces `codesigned.SHA256SUMS`. Since `all.SHA256SUMS` makes more sense (as the file contains all the sha256sums, not just the codesigned ones), `guix-attest` has been changed to output a file of that name.

  As a quality of life improvement, `guix-verify` can take `SIGNER` and use the signer's manifest as the base to compare against. This makes it easier to compare a single person's attestations with everyone else's and can make it more obvious when one builder is clearly mismatching with everyone else.

  Lastly `release-process.md` is updated with a note about a gotcha that can cause a mismatch in the codesigned attestation.

ACKs for top commit:
  fanquake:
    ACK 9b313dfef1

Tree-SHA512: 0d60627def38288dbd3059ad1e72cad224f9205da11b1a561c082ef28250a074df5cc5f2797c91a7be027bc486a3fda3319c2e496a8724e5b539337236c6f990

contrib/guix/guix-build

@@ -232,7 +232,7 @@ host_to_commonname() {
 }
 
 # Determine the reference time used for determinism (overridable by environment)
-SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git log --format=%at -1)}"
+SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git -c log.showSignature=false log --format=%at -1)}"
 
 # Execute "$@" in a pinned, possibly older version of Guix, for reproducibility
 # across time.