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".

docs/build_manpage.py

@@ -1,13 +1,10 @@
 #!/usr/bin/env python3
-from datetime import date
 from io import StringIO
 from lxml import etree
 import markdown
 from markdown.extensions import Extension
 import re
-import os
 import sys
-from time import time
 import yaml
 
 
@@ -68,9 +65,7 @@ class RoffWalker(object):
         last_tag = None
         while root is not None:
             if root.tag == 'h1':
-                epoch = int(os.environ.get('SOURCE_DATE_EPOCH', time()))
-                self.__write_cmd('.TH "JQ" "1" "{}" "" ""'.format(
-                    date.fromtimestamp(epoch).strftime('%B %Y')))
+                self.__write_cmd('.TH "JQ" "1" "" "" ""')
                 self.__write_cmd('.SH "NAME"')
                 # TODO: properly parse this
                 self.__write_raw(r'\fBjq\fR \- Command\-line JSON processor' +