sstate: Fix sstate file mirroring checksum issue

After a recent fix in bitbake, the checksums may be checked for local file
fetches at an earlier code point. The underlying recipe checksums won't
match sstate values so ensure these aren't set via SRC_URI flags when
reconfiguring the fetch for sstate.

(From OE-Core rev: 315cfebe77a46c868f307919d3540046a6cb8587)

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

meta/classes-global/sstate.bbclass

@@ -784,6 +784,7 @@ def pstaging_fetch(sstatefetch, d):
         uris += ['file://{0}.sig;downloadfilename={0}.sig'.format(sstatefetch)]
 
     for srcuri in uris:
+        localdata.delVar('SRC_URI')
         localdata.setVar('SRC_URI', srcuri)
         try:
             fetcher = bb.fetch2.Fetch([srcuri], localdata, cache=False)