pathInfoIsTrusted -> pathInfoIsUntrusted

I guess the rationale behind the old name wath that
`pathInfoIsTrusted(info)` returns `true` iff we would need to `blindly`
trust the path (because it has no valid signature and `requireSigs` is
set), but I find it to be a really confusing footgun because it's quite
natural to give it the opposite meaning.

src/libstore/store-api.hh

@@ -384,7 +384,7 @@ public:
        we don't really want to add the dependencies listed in a nar info we
        don't trust anyyways.
        */
-    virtual bool pathInfoIsTrusted(const ValidPathInfo &)
+    virtual bool pathInfoIsUntrusted(const ValidPathInfo &)
     {
         return true;
     }