`./x.py` fails to bootstrap if `build.vendor` is set in `config.toml`

[yungkneez]

Steps to reproduce:

1. Clone the repository

git clone https://github.com/rust-lang/rust.git
cd rust
cp config.toml.example config.toml

2. Modify config.toml to set build.vendor = true

3. Start the bootstrap:

./x.py help

I expected to see this happen: x.py builds the bootstrap and runs the subcommand

Instead, this happened:

error: failed to read `<hidden>/rust/./src/tools/rust-analyzer/Cargo.toml`

Caused by:
    No such file or directory (os error 2)
failed to run: <hidden>/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo vendor --sync=./src/tools/rust-analyzer/Cargo.toml --sync=./compiler/rustc_codegen_cranelift/Cargo.toml

Cause

rust-analyzer is not checked out in src/bootstrap/bootstrap.py:

rust/src/bootstrap/bootstrap.py

Lines 1142 to 1149 in 082e4ca

	submodules = [
	"src/tools/rust-installer",
	"src/tools/cargo",
	"src/tools/rls",
	"src/tools/miri",
	"library/backtrace",
	"library/stdarch"
	]