lib/oe/gpg_sign.py: Avoid race when creating .sig files in detach_sign

lib/oe/gpg_sign.py: Avoid race when creating .sig files in detach_sign

Move the signature file into place only after it is successfully signed.
This to avoid race and corrupted .sig files in cases multiple onging
builds write to a shared sstate-cache dir.

Signed-off-by: Tobias Hagelborn <tobiasha@axis.com>
Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b4ec08ea9efebac262d43f47d95a356fe2829de9)
Signed-off-by: Steve Sakoman <steve@sakoman.com>