classes/reproducible_build: Use atomic rename for SDE file

If an existing source date epoch file was found during do_unpack, it was
deleted and a new one would be written in its place. This causes a race
with check-before-use code in get_source_date_epoch_value. Resolve the
problem by making do_unpack write the new source date epoch to a
temporary file, then do an atomic rename to ensure it's always present,
and change the check-before-use code to use a EAFP exception instead of
checking for file existence.

[YOCTO #14384]

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>