Further build-many-glibcs.py fixes for utcnow() deprecation

It turns out that the replacement of datetime.datetime.utcnow(), for a
warning produced early in running build-many-glibcs.py with Python
3.12, (a) wasn't complete (there were other uses elsewhere in the
script also needing updating) and (b) broke reading of build-time from
build-state.json, because an aware datetime was written out including
+00:00 for the timezone, which was not expected by the strptime call.

Fix the first by making the change to
datetime.datetime.now(datetime.timezone.utc) for all the remaining
utcnow() calls.  Fix the second by using strftime with an explicit
format instead of just str() when formatting build times for
build-state.json and and email subjects, and then setting the timezone
explicitly when reading from build-state.json.  (Other uses, in
particular messages output by the bot, continue to use str() as the
precise format should not matter in those cases; it shouldn't actually
matter for email subjects either but it seems a good idea to keep
those short.)

Tested with a bot-cycle run and checking the format of times in
build-state.json afterwards.

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