Remove signRealisation from drv goal

We can move this method from `LocalStore` to `Store` --- even if we only
want the actual builder to sign things in many cases, there is no reason
to try to enforce this policy by spurious moving the method to a
subclass.

Now, we might technically sign class, but CA derivations is
experimental, and @Ericson2314 is going to revisit all this stuff with
issue #11896 anyways.

src/libstore/store-api.hh

@@ -622,6 +622,8 @@ public:
     virtual void addSignatures(const StorePath & storePath, const StringSet & sigs)
     { unsupported("addSignatures"); }
 
+    void signRealisation(Realisation &);
+
     /* Utility functions. */
 
     /**