Rollup merge of #73317 - davidtwco:bootstrap-config-env-var, r=Mark-Simulacrum

bootstrap: read config from $RUST_BOOTSTRAP_CONFIG

This PR modifies bootstrap so that `config.toml` is read first from `RUST_BOOTSTRAP_CONFIG`, then `--config` and finally `config.toml` in the current directory.

This is a subjective change, intended to improve the ergnomics when using "development shells" for rustc development (for example, using tools such as Nix) which set environment variables to ensure a reproducible environment (these development shells can then be version controlled, e.g. [my rustc shell](https://github.com/davidtwco/veritas/blob/6b74a5c170b6efb2c7b094352932f9158f97eec0/nix/shells/rustc.nix)). By optionally reading `config.toml` from an environment variable, a `config.toml` can be defined in the development shell and a path to it exposed in the `RUST_BOOTSTRAP_CONFIG` environment variable - avoiding the need to manually symlink the contents of this file to `config.toml` in the working directory.

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