aurb: keep ThinLTO cache entries until the size cap

lld expires entries unused for a week by default, which empties the cache between most package rebuilds; prune_after=0s leaves only the 10 GB cap.

.config/aurb/clang.cfg

@@ -2,4 +2,4 @@
 # Here rather than in LDFLAGS: old libtool (<2.4.7) drops -fuse-ld=lld, and these then reach GNU ld.
 -fuse-ld=lld
 -Wl,--thinlto-cache-dir=/home/fuxino/.cache/thinlto
--Wl,--thinlto-cache-policy=cache_size_bytes=10g
+-Wl,--thinlto-cache-policy=cache_size_bytes=10g:prune_after=0s