sstate: Avoid problems with recipes using SRCPV when fetching sstate

When fetching sstate, SRCPV can be expanded withuin PV which breaks the
sstate fetcher as other variables like SRC_URI are changed by the fetcher
code. Expand the variable before doing this to avoid those problems and
hence avoid setscene task failures with remote sstate storage in git
recipes.

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