Improve panic message on "Found multiple rlibs" error in compile-test

[popzxc]

Related to #7343

When I first met this error I was pretty much confused, so I thought it may be a good idea to:

• Give a hint on what to do to users that don't want to dig into specifics and just want to quickly resolve the issue.
• Give a link for those who are interested in details.

Old appearance:

New appearance:

Please write a short comment explaining your change (or "none" for internal only changes)

changelog: none

[rust-highfive]

r? @giraffate

(rust-highfive has picked a reviewer for you, use r? to override)

[jyn514]

cargo clean will do nothing in rust-lang/rust. Can you suggest "Remove the stageN-tools directory" instead when RUSTC_BOOTSTRAP is set in the environment? (or any other way of detecting x.py; BOOTSTRAP_PYTHON should work too)

[flip1995]

@bors r+

Thanks!

[bors]

📌 Commit 310a204 has been approved by flip1995

[bors]

⌛ Testing commit 310a204 with merge d3327bd...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing d3327bd to master...