Current beta cannot bootstrap itself

[Keruspe]

When trying to build the current beta (2bc1d40) using itself, the build of bootstrap fails with

error: the item `Seek` is imported redundantly
    --> src/bootstrap/lib.rs:1128:23
     |
126  | use std::io::{Seek, SeekFrom, Write, Read};
     |               ---- the item `Seek` is already imported here
...
1128 |         use std::io::{Seek, SeekFrom};
     |                       ^^^^
     |
note: lint level defined here
    --> src/bootstrap/lib.rs:107:9
     |
107  | #![deny(warnings)]
     |         ^^^^^^^^
     = note: #[deny(unused_imports)] implied by #[deny(warnings)]

error: the item `SeekFrom` is imported redundantly
    --> src/bootstrap/lib.rs:1128:29
     |
126  | use std::io::{Seek, SeekFrom, Write, Read};
     |                     -------- the item `SeekFrom` is already imported here
...
1128 |         use std::io::{Seek, SeekFrom};
     |                             ^^^^^^^^

error: aborting due to 2 previous errors

error: Could not compile `bootstrap`.

[hellow554]

@rustbot modify labels: A-rustbuild

also label:regression-from-stable-to-beta

[rustbot]

Error: Label regression-from-stable-to-beta can only be set by Rust team members

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[emilyalbini]

@rustbot modify labels: T-infra

I'm not sure we support bootstrapping a beta with itself. @Mark-Simulacrum?

[mati865]

Once beta is promoted to stable it will be bootstrapping stable with stable which Linux distributions do a lot so it'd be nice.

[Mark-Simulacrum]

We try to support distro's efforts to support self-bootstrapping; the PR (#60273) already posted looks fine to me. I'll leave a comment to that effect there as well.