sstate-cache-management: fix regex for 'sigdata' stamp files

sstate-cache-management: fix regex for 'sigdata' stamp files

Given file names like
    6.4.do_fetch.sigdata.821b6c62f9f2bd8b7e1378656b8319697a21f6f6e4a351f98dc325a18ef7ed0f,
I'm pretty sure we want to match the dot here, not any character.

Fixes: 2fa1b25d7485 ("sstate-cache-management: Rewrite in python") in oe-core
Fixes: b723fcaac52f ("sstate-cache-management: Rewrite in python") in poky
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>