Reserve the __contentAddressed derivation parameter

Not implementing anything here, just throwing an error if a derivation
sets `__contentAddressed = true` without
`--experimental-features content-addressed-paths`
(and also with it as there's nothing implemented yet)

src/libstore/parsed-derivations.hh

@@ -34,6 +34,8 @@ public:
     bool willBuildLocally() const;
 
     bool substitutesAllowed() const;
+
+    bool contentAddressed() const;
 };
 
 }