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.

python/servo/platform/linux.py

@@ -150,17 +150,6 @@ class Linux(Base):
         installed_something |= self._platform_bootstrap_gstreamer(force)
         return installed_something
 
-    def linker_flag(self) -> str:
-        # the rust-lld binary downloaded by rustup
-        # doesn't respect NIX_LDFLAGS and also needs
-        # other patches to work correctly. Use system
-        # version of lld for now. See
-        # https://github.com/NixOS/nixpkgs/issues/220717
-        if self.distro.lower() == 'nixos':
-            return '-C link-arg=-fuse-ld=lld'
-        else:
-            return '-Zgcc-ld=lld'
-
     def install_non_gstreamer_dependencies(self, force: bool) -> bool:
         install = False
         pkgs = []