Make timestamps in manual pages reproducible

Refer to https://reproducible-builds.org/docs/source-date-epoch/ for
documentation on SOURCE_DATE_EPOCH.

Bug-Debian: https://bugs.debian.org/1029807
Signed-off-by: Helmut Grohne <helmut@subdivi.de>

(cherry picked from commit 98352cf667)

setup_docs.py

@@ -7,6 +7,7 @@ import sys
 import textwrap
 from collections import OrderedDict
 from datetime import datetime
+import time
 
 from setuptools import Command
 
@@ -459,7 +460,10 @@ class build_man(Command):
         self.write_heading(write, description, double_sided=True)
         # man page metadata
         write(':Author: The Borg Collective')
-        write(':Date:', datetime.utcnow().date().isoformat())
+        write(
+            ':Date:',
+            datetime.utcfromtimestamp(int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))).date().isoformat(),
+        )
         write(':Manual section: 1')
         write(':Manual group: borg backup tool')
         write()