bootstrap: temporarily flip compile::Rustc vs compile::Assemble

The PathSet prefix matching unfortunately also has implications for `./x
build compiler`, because the path filter `"compiler"` gets consumed by
`compile::Rustc` step first after PathSet prefix matching, whereas
before PathSet prefix matching, the later-registered `compile::Assemble`
step would've consumed the `"compiler"` path filter.

This merely papers over the issue with PathSet prefix handling to
unblock contributors for using `./x build compiler`.

Author: jieyouxu

src/bootstrap/src/core/builder/mod.rs

@@ -825,8 +825,12 @@ impl<'a> Builder<'a> {
         match kind {
             Kind::Build => describe!(
                 compile::Std,
-                compile::Rustc,
+                // FIXME(#135022): `compile::Assemble` **must** come before `compile::Rustc` after
+                // `PathSet` also permits prefix-matching, because `compile::Rustc` can consume the
+                // `"compiler"` path filter first, causing `compile::Assemble` to no longer run when
+                // the user writes `./x build compiler --stage 0`.
                 compile::Assemble,
+                compile::Rustc,
                 compile::CodegenBackend,
                 compile::StartupObjects,
                 tool::BuildManifest,