Remove lingering forced nightly usages

These look to have been removed elsewhere, so seem to be left in by
mistake!

src/test/mod.rs

@@ -30,7 +30,7 @@ where
     let output = {
         let mut cmd = Command::new("cargo");
         cmd.envs(envs);
-        cmd.current_dir(path).arg("+nightly").arg("test");
+        cmd.current_dir(path).arg("test");
         if release {
             cmd.arg("--release");
         }

tests/all/utils/fixture.rs

@@ -253,7 +253,6 @@ impl Fixture {
     pub fn cargo_check(&self) -> &Self {
         Command::new("cargo")
             .current_dir(&self.path)
-            .arg("+nightly")
             .arg("check")
             .arg("--target")
             .arg("wasm32-unknown-unknown")