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/shared/script/compositor.rs

@@ -97,7 +97,7 @@ impl ScrollTreeNode {
         &mut self,
         scroll_location: ScrollLocation,
     ) -> Option<(ExternalScrollId, LayoutVector2D)> {
-        let mut info = match self.scroll_info {
+        let info = match self.scroll_info {
             Some(ref mut data) => data,
             None => return None,
         };