sstate: Use SSTATE_DIR for FILESPATH

sstate: Use SSTATE_DIR for FILESPATH

FILESPATH was only being overridden in one fetch location, it should be
equally handled in both.

Also use SSTATE_DIR as FILESPATH so that mirror urls which do remapping
can search the local SSTATE_DIR for other paths.

Also ensure that MIRRORS is removed in both locations, previously
it was only unset in one but both codepaths should be consistent.

(From OE-Core rev: ab6bebddbdefec323e284b6438d9c57b3d8a2cc3)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>