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/build.cc

@@ -809,6 +809,9 @@ private:
     /* Whether this is a fixed-output derivation. */
     bool fixedOutput;
 
+    /* Whether this is a content adressed derivation */
+    bool contentAddressed = false;
+
     /* Whether to run the build in a private network namespace. */
     bool privateNetwork = false;
 
@@ -1195,6 +1198,14 @@ void DerivationGoal::haveDerivation()
 
     parsedDrv = std::make_unique<ParsedDerivation>(drvPath, *drv);
 
+    contentAddressed = parsedDrv->contentAddressed();
+
+    if (this->contentAddressed) {
+        settings.requireExperimentalFeature("content-addressed-paths");
+        throw Error("content-addressed-paths isn't implemented yet");
+    }
+
+
     /* We are first going to try to create the invalid output paths
        through substitutes.  If that doesn't work, we'll build
        them. */