guix: Ensure EPOCH_SOURCE_DATE does not include GPG information

If the user has set log.showSignature=true in their git config, then the
git log will always output GPG signature information. Since git log is
used to set EPOCH_SOURCE_DATE, this will mistakenly have GPG signature
information in it which causes issues for the build. To avoid this
issue, we override the config and force log.showSignature=false.

contrib/guix/guix-codesign

@@ -220,7 +220,7 @@ fi
 JOBS="${JOBS:-$(nproc)}"
 
 # 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.