Only build, but don't run tests in MSRV CI (#2558)

* Only build, but don't run tests in MSRV CI

Since the MSRV of development dependencies can easily be bumped without it affecting the MSRV of the published version of `winit`

* Run clippy on stable Rust instead of MSRV Rust

clippy inspects the `rust-version` field, and only suggests changes that conform to that.

src/platform_impl/linux/x11/mod.rs

@@ -644,7 +644,7 @@ impl Deref for Window {
     type Target = UnownedWindow;
     #[inline]
     fn deref(&self) -> &UnownedWindow {
-        &*self.0
+        &self.0
     }
 }