Bump fetcher cache version

We're getting more reports in https://github.com/NixOS/nix/issues/10985

It appears that something hasn't gone right process-wise.
I find this mistake not to be worth investigating, but rather something
to pay attention to going forward.

Let's nip this in the bud.

Closes https://github.com/NixOS/nix/issues/10985

src/libfetchers/cache.cc

@@ -36,7 +36,7 @@ struct CacheImpl : Cache
     {
         auto state(_state.lock());
 
-        auto dbPath = getCacheDir() + "/fetcher-cache-v2.sqlite";
+        auto dbPath = getCacheDir() + "/fetcher-cache-v3.sqlite";
         createDirs(dirOf(dbPath));
 
         state->db = SQLite(dbPath);