Make nix-shell support content-addressed derivations

Resolve the derivation before trying to load its environment −
essentially reproducing what the build loop does − so that we can
effectively access our dependencies (and not just their placeholders).

Fix #4821

src/nix-build/nix-build.cc

@@ -387,6 +387,13 @@ static void main_nix_build(int argc, char * * argv)
 
         if (dryRun) return;
 
+        if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+            auto resolvedDrv = drv.tryResolve(*store);
+            if (!resolvedDrv)
+                throw Error("unable to resolve the derivation '%s'. nix-shell can’t continue", drvInfo.queryDrvPath());
+            drv = *resolvedDrv;
+        }
+
         // Set the environment.
         auto env = getEnv();