servo: Merge #15875 - Fix bug bootstrapping stable rust (from mbrubeck:bootstrap-stable); r=nox

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they touch build infrastructure only

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Source-Repo: https://github.com/servo/servo
Source-Revision: 2ad2e095719bc489e9902a64fccddffe2c5662d2

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 470bcab7513f3ddda1526fc25b53c5d83b77ae40

servo/python/servo/bootstrap_commands.py

@@ -71,7 +71,8 @@ class MachCommands(CommandBase):
         rust_dir = path.join(self.context.sharedir, "rust", rust_path)
         install_dir = path.join(self.context.sharedir, "rust", version)
         if not self.config["build"]["llvm-assertions"]:
-            install_dir += "-alt"
+            if not self.use_stable_rust():
+                install_dir += "-alt"
 
         if not force and path.exists(path.join(rust_dir, "rustc", "bin", "rustc" + BIN_SUFFIX)):
             print("Rust compiler already downloaded.", end=" ")