separate the crates to test from the test setup

it'll be easier to see and update the list: the other cmd args
can just be ignored

tests/run-make/rustc-crates-on-stable/rmake.rs

@@ -25,7 +25,9 @@ fn main() {
                 // folder
                 "--target-dir",
                 ".",
-                // Check that these crates can be compiled on "stable"
+            ])
+            // Check that these crates can be compiled on "stable"
+            .args(&[
                 "-p",
                 "rustc_type_ir",
                 "-p",