Cannot run `x.py` under sudo as non-root user

[chbaker0]

I am trying to build Rust from source on my project's continuous integration system. I see the following error:

info: looks like you are running this command under `sudo`
      and so in order to preserve your $HOME this will now
      use vendored sources by default.
error: vendoring required, but vendor directory does not exist.
       Run `cargo vendor` without sudo to initialize the vendor directory.

Looking at the message and #40108 it seems the intent is to prevent putting root-owned files in a non-root user's home directory.

Our CI builders use sudo to step down from root to a sandboxed user. See https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket/8823996676604838337/+/u/build_rust/execution_details for an example environment. I believe it should be safe to build without vendoring in this case.

bootstrap.py should skip the check if SUDO_USER is not root, or an option to bypass this check should be provided.

[Mark-Simulacrum]

If the implementation for this is relatively straightforward, a PR is likely to be accepted.

[jyn514]

@rustbot label; +E-easy +E-help-wanted

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[jyn514]

@rustbot label: +E-easy +E-help-wanted

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