bootstrap: minimal fix for ./x install src with build.docs = false

When running the `install src` command I'm seeing failures as the
`builder.doc_out(host)` directory does not exist. This is because
`match_paths_to_steps_and_run()` doesn't actually build any
documentation as the `paths.is_empty()` causes an early return. This
results in install failures as the `*/doc` src directory doesn't exist.

This patch ensures that the builder.doc_out(host) directory exists.

This fixes installing the Rust source when `build.docs = false`.

This fixes installing the Rust source code in OpenEmbedded.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Jieyou Xu <jieyouxu@outlook.com>

Author: alistair23

src/bootstrap/src/core/build_steps/dist.rs

@@ -87,6 +87,12 @@ impl Step for Docs {
         // from a shared directory.
         builder.run_default_doc_steps();
 
+        // In case no default doc steps are run for host, it is possible that `<host>/doc` directory
+        // is never created.
+        if !builder.config.dry_run() {
+            t!(fs::create_dir_all(builder.doc_out(host)));
+        }
+
         let dest = "share/doc/rust/html";
 
         let mut tarball = Tarball::new(builder, "rust-docs", &host.triple);