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/tests/compositor.rs

@@ -155,7 +155,7 @@ fn test_scroll_tree_chain_through_overflow_hidden() {
         .get_node_mut(&overflow_hidden_id)
         .scroll_info
         .as_mut()
-        .map(|mut info| {
+        .map(|info| {
             info.scroll_sensitivity = ScrollSensitivity::Script;
         });