x.py fails if .cargo/config configures `build.target`

### Summary



When `$CARGO_HOME/config.toml` sets [`build.target`](https://doc.rust-lang.org/cargo/reference/config.html#buildtarget), `x` fails to launch bootstrap.

### Command used

```sh
./x.py setup
```

### Expected behaviour



Setup would run as usual.

### Actual behaviour



`x.py` tries and fails to launch `{workspace-root}/build/bootstrap/debug/bootstrap`. The `bootstrap` binary was actually generated at (in my case) `{workspace-root}/build/bootstrap/{host-tuple}/debug/bootstrap`.

### Bootstrap configuration (bootstrap.toml)

n/a

### Operating system



Ubuntu 25.10

### HEAD



cf1817bc6ecd2d14ca492247c804bad31948dd56

### Additional context


My `$CARGO_HOME/config.toml`:

```toml
[build]
target = ["host-tuple"]
build-dir = "{cargo-cache-home}/build/{workspace-path-hash}"

[install]
root = "~/.local" # /bin implicit
```


<details><summary>Build Log</summary>
<p>

```txt
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.06s
Traceback (most recent call last):
  File "/home/cad/src/rust-lang/rust/x.py", line 53, in <module>
    bootstrap.main()
    ~~~~~~~~~~~~~~^^
  File "/home/cad/src/rust-lang/rust/src/bootstrap/bootstrap.py", line 1425, in main
    bootstrap(args)
    ~~~~~~~~~^^^^^^
  File "/home/cad/src/rust-lang/rust/src/bootstrap/bootstrap.py", line 1383, in bootstrap
    run(args, env=env, verbose=build.verbose, is_bootstrap=True)
    ~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/cad/src/rust-lang/rust/src/bootstrap/bootstrap.py", line 232, in run
    ret = subprocess.Popen(args, **kwargs)
  File "/usr/lib/python3.13/subprocess.py", line 1039, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                        pass_fds, cwd, env,
                        ^^^^^^^^^^^^^^^^^^^
    ...<5 lines>...
                        gid, gids, uid, umask,
                        ^^^^^^^^^^^^^^^^^^^^^^
                        start_new_session, process_group)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/subprocess.py", line 1857, in _execute_child
    self._posix_spawn(args, executable, env, restore_signals, close_fds,
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                      p2cread, p2cwrite,
                      ^^^^^^^^^^^^^^^^^^
                      c2pread, c2pwrite,
                      ^^^^^^^^^^^^^^^^^^
                      errread, errwrite)
                      ^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/subprocess.py", line 1801, in _posix_spawn
    self.pid = os.posix_spawn(executable, args, env, **kwargs)
               ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/home/cad/src/rust-lang/rust/build/bootstrap/debug/bootstrap'
```

</p>
</details>

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Uh oh!

x.py fails if .cargo/config configures `build.target` #155651

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Expected behaviour

Actual behaviour

Bootstrap configuration (bootstrap.toml)

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Uh oh!

x.py fails if .cargo/config configures build.target #155651

Description

Summary

Command used

Expected behaviour

Actual behaviour

Bootstrap configuration (bootstrap.toml)

Operating system

HEAD

Additional context

Metadata

Metadata

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Labels

Type

Fields

Projects

Milestone

Relationships

Development

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x.py fails if .cargo/config configures `build.target` #155651