Compile Servo with the latest version of rust stable (#30831)

This completes the transition to compiling Servo with rust stable. Some
nightly-only features are still used when compiling the `script` and
`crown` crates, as well as for some style unit tests. These will likely
break with newer compiler versions, but `crown` can be disabled for them
conditionally. This is just the first step.

This has some caveats:

1. We need to disable setting up the special linker on Linux. The -Z
   option isn't supported with stable rust so using this is out --
   meanwhile we can't be sure that lld is installed on most systems.
2. `cargo fmt` still uses some unstable options, so we need to rely on
   the unstable toolchain just for running `fmt`. The idea is to fix this
   gradually.

components/layout_thread/lib.rs

@@ -1241,7 +1241,7 @@ impl LayoutThread {
         );
 
         // Flush shadow roots stylesheets if dirty.
-        document.flush_shadow_roots_stylesheets(&mut self.stylist, guards.author.clone());
+        document.flush_shadow_roots_stylesheets(&mut self.stylist, guards.author);
 
         let restyles = std::mem::take(&mut data.pending_restyles);
         debug!("Draining restyles: {}", restyles.len());