Drop the date from the committed man page (#3659)

The date in the .TH line of jq.1.prebuilt changed whenever the manual was
regenerated in a new month, causing merge conflicts between pull requests
that touch the manual, and it did not reflect the release date anyway.
Generate the committed man page without a date.

Stamp the date into jq.1 at build time instead, taking it from the current
commit, or from SOURCE_DATE_EPOCH where that is set, so that release
tarballs and distribution builds still carry one and the behaviour added
in #3514 is kept. Write it in UTC and in ISO 8601 rather than the previous
"%B %Y", for which mandoc reports "cannot parse date, using it verbatim".

.github/workflows/manpage.yml

@@ -42,12 +42,8 @@ jobs:
           ./configure --enable-docs
       - name: Build man page, man.test, manonig.test
         run: |
-          mv jq.1.prebuilt jq.1.old
-          rm -f tests/man.test manonig.test
+          rm -f jq.1.prebuilt tests/man.test tests/manonig.test
           make jq.1.prebuilt tests/man.test tests/manonig.test
       - name: Make sure that jq.1.prebuilt, man.test, manonig.test are up to date
         run: |
-          git diff --exit-code tests/man.test tests/manonig.test
-          # skip build date in jq.1.prebuilt
-          test -s jq.1.prebuilt
-          diff -- <(tail -n +3 jq.1.old) <(tail -n +3 jq.1.prebuilt)
+          git diff --exit-code jq.1.prebuilt tests/man.test tests/manonig.test