Fix spec validation & merge so that default values are correctly applied

Signed-off-by: Solomon Hykes <sh.github.6811@hykes.org>

examples/tests/exec/env/overlay/main.cue

@@ -14,7 +14,5 @@ bar: string
 				[ "$foo" == "overlay environment" ] || exit 1
 			"""]
 		env: foo: bar
-		// XXX Blocked by https://github.com/blocklayerhq/dagger/issues/19
-		dir: "/"
 	},
 ]