feat: Enable shared cache for proguard

There wasn't much point sharing proguard files via
the shared cache because we were downloading them
exclusively from Sentry anyway. But now we don't use
actual proguard files anymore, we compute cache files
from them and use those. So it makes sense to share
the cache files between Symbolicators.

crates/symbolicator-proguard/src/service.rs

@@ -185,6 +185,6 @@ impl CacheItemRequest for FetchProguard {
     }
 
     fn use_shared_cache(&self) -> bool {
-        false
+        true
     }
 }