mark rust-analyzer as a host-only tool

All tools meant to be shipped with host toolchains only should be marked
as `ONLY_HOSTS = true`, but rust-analyzer was marked as `ONLY_HOSTS =
false` incorrectly. This meant that bootstrap attempted to build
rust-analyzer for cross-compilation-only targets, causing errors because
libstd is not present on some of them.

It will still be possible to cross-compile rust-analyzer by passing a
different --host flag to ./x, like you can cross-compile other tools.

Author: pietroalbini

src/bootstrap/tool.rs

@@ -698,7 +698,7 @@ pub struct RustAnalyzer {
 impl Step for RustAnalyzer {
     type Output = Option<PathBuf>;
     const DEFAULT: bool = true;
-    const ONLY_HOSTS: bool = false;
+    const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let builder = run.builder;
@@ -742,7 +742,7 @@ pub struct RustAnalyzerProcMacroSrv {
 impl Step for RustAnalyzerProcMacroSrv {
     type Output = Option<PathBuf>;
     const DEFAULT: bool = true;
-    const ONLY_HOSTS: bool = false;
+    const ONLY_HOSTS: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let builder = run.builder;