Expected format is `%Y-%m-%d %H:%M%z`.
Previous code would not generate the expected format of timestamps for
PO files headers (specifically in the the `POT-Creation-Date` field).
This could lead to some parsers (like e.g. the `sphinx_intl.load_po`
function) failing to load these PO files.
Note: since `datetime` module requires additional gymnastic to get valid
timezone info,switched back to the simpler `time` module here.
All times are expressed in UTC (aka GMT) timezone, since these are also
the values stored for Blender buildtimes.
Reported by @hoanguk in the Blender translations chat, thanks.
f9c3d51322