Relegate toRealPath to LocalFSStore

Fix #14480

This method is not well-defined for arbitrary stores, which do not have
a notion of a "real path" -- it is only well-defined for local file
systems stores, which do have exactly that notion, and so it is moved to
that sub-interface instead.

Some call-sites had to be fixed up for this, but in all cases the
changes are positive. Using `getFSSourceAccessor` allows for more other
stores to work properly. `nix-channel` was straight-up wrong in the case
of redirected local stores. And the building logic with remote building
and a non-local store is also fixed, properly gating some deletions on
store type.

Co-authored-by: Robert Hensing <robert@roberthensing.nl>

src/libstore/local-overlay-store.cc

@@ -246,7 +246,7 @@ void LocalOverlayStore::optimiseStore()
         if (lowerStore->isValidPath(path)) {
             uint64_t bytesFreed = 0;
             // Deduplicate store path
-            deleteStorePath(Store::toRealPath(path), bytesFreed);
+            deleteStorePath(toRealPath(path), bytesFreed);
         }
         done++;
         act.progress(done, paths.size());