bootstrap seems to default to downloading the LLVM submodule

Summary

Run the following (in a GH CI job):

git clone https://github.com/rust-lang/rust
cd rust
./x check miri

Expected behaviour

The bootstrap.example.toml indicates that a prebuilt LLVM is downloaded by default. That's also what happens when I run these commands locally.

Actual behaviour

In CI, I see:

Updating submodule src/llvm-project
Submodule 'src/llvm-project' (https://github.com/rust-lang/llvm-project.git) registered for path 'src/llvm-project'
Cloning into '/home/runner/work/miri/rust/src/llvm-project'...
remote: Enumerating objects: 3, done.        
remote: Counting objects:  33% (1/3)        
remote: Counting objects:  66% (2/3)        
remote: Counting objects: 100% (3/3)        
remote: Counting objects: 100% (3/3), done.        
remote: Compressing objects:  50% (1/2)        
remote: Compressing objects: 100% (2/2)        
remote: Compressing objects: 100% (2/2), done.        
remote: Total 3 (delta 0), reused 3 (delta 0), pack-reused 0 (from 0)        
Receiving objects:  33% (1/3)
Receiving objects:  66% (2/3)
Receiving objects: 100% (3/3)
Receiving objects: 100% (3/3), done.
From https://github.com/rust-lang/llvm-project
 * branch                333793696b0a08002acac6af983adc7a5233fc56 -> FETCH_HEAD
Submodule path 'src/llvm-project': checked out '333793696b0a08002acac6af983adc7a5233fc56'

This takes close to 2 minutes and wastes a bunch of bandwidth.

Bootstrap configuration (bootstrap.toml)

(none)

Operating system

ubuntu-latest

HEAD

2f4dfc753fd86c672aa4145940db075a8a149f17

Start with bootstrap.example.toml and the bootstrap path used by ./x check miri, then compare its LLVM selection in local runs and the GH CI environment. Check when src/llvm-project is initialized and verify that the expected prebuilt LLVM behavior avoids the unnecessary submodule download.