sstate: apply proper umask when fetching from SSTATE_MIRROR

sstate: apply proper umask when fetching from SSTATE_MIRROR

Currently, files and directories created under ${SSTATE_DIR} when
fetching from an sstate mirror are not created with group write,
unlike when the sstate artifacts are generated locally. That's
inconsistent, and problematic when the local sstate dir is shared
among multiple users.

Wrap the fetching in a bb.utils.umask() context manager, and for simplicity
move the mkdir of SSTATE_DIR inside that.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>