message: contrib: guix: Clarify SOURCE_DATE_EPOCH.
diff: @@ -110,9 +110,9 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
  * _**SOURCE_DATE_EPOCH**_
  - Override the reference timestamp used for bit-for-bit reproducibility, the
  - variable name conforms to [standard][r12e/source-date-epoch]. _(defaults to
  - the output of `$(git log --format=%at -1)`)_
  + Override the reference UNIX timestamp used for bit-for-bit reproducibility,
  + the variable name conforms to [standard][r12e/source-date-epoch]. _(defaults
  + to the output of `$(git log --format=%at -1)`)_
  * _**V**_
  