Bump fetcher cache version

We're getting more reports in #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 #10985

(cherry picked from commit e948c8e)

# Conflicts:
#	src/libfetchers/cache.cc

src/libfetchers/cache.cc

@@ -36,7 +36,11 @@ struct CacheImpl : Cache
     {
         auto state(_state.lock());
 
+<<<<<<< HEAD
         auto dbPath = getCacheDir() + "/nix/fetcher-cache-v2.sqlite";
+=======
+        auto dbPath = getCacheDir() + "/fetcher-cache-v3.sqlite";
+>>>>>>> e948c8e03 (Bump fetcher cache version)
         createDirs(dirOf(dbPath));
 
         state->db = SQLite(dbPath);