release.sh: put vdir in xmtmp, not XBMK_CACHE

XBMK_CACHE is meant for permanent cached files, not
temporarily files.

the temporary release files are copied upon successful
return, to their rightful place under release/

this new change also reduces the chance of race
conditions, if multiple xbmk instances are used; while
not yet supported as a use-case, this is a goal for a
future design change.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/release.sh

@@ -21,7 +21,9 @@ release()
 	[ -e "$reldest" ] && \
 	    err "already exists: \"$reldest\""
 
-	vdir="$XBMK_CACHE/relpwd/${xbtmp##*/}/$version"
+	vdir="`mktemp -d || err "can't make vdir"`" || err
+	vdir="$vdir/$version"
+
 	rsrc="$vdir/${relname}_src"
 
 	remkdir "$vdir"