Respect RUST_BACKTRACE for delayed bugs

[Noratrieb]

Sometimes, especially with MIR validation, the backtraces from delayed bugs are noise and make it harder to look at them. Respect the environment variable and don't print it when the user doesn't want it.

[rustbot]

r? @oli-obk

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[Noratrieb]

it worked when i tested it at first but now it doesn't work again, somethings wrong

[Noratrieb]

it works just fine, i just held it wrong.
example:

error: internal compiler error: broken MIR in Item(DefId(0:21 ~ uwu[d045]::{impl#0}::deserialize)) (after pass PreCodegen) at bb0[1]:
                                Field projection `(*_2).0` specified type `u32`, but actual type is `<[<T as Archive>::Archived] as ArchivePointee>::ArchivedMetadata`
  --> uwu.rs:21:57
   |
21 |     let _a: <[T] as ArchivePointee>::ArchivedMetadata = v.metadata;
   |                                                         ^^^^^^^^^^
   |
   = note: delayed at compiler/rustc_const_eval/src/transform/validate.rs:354:30 - disabled backtrace

error: internal compiler error: broken MIR in Item(DefId(0:21 ~ uwu[d045]::{impl#0}::deserialize)) (after phase change to runtime-optimized) at bb0[1]:
                                Field projection `(*_2).0` specified type `u32`, but actual type is `<[<T as Archive>::Archived] as ArchivePointee>::ArchivedMetadata`
  --> uwu.rs:21:57
   |
21 |     let _a: <[T] as ArchivePointee>::ArchivedMetadata = v.metadata;
   |                                                         ^^^^^^^^^^
   |
   = note: delayed at compiler/rustc_const_eval/src/transform/validate.rs:354:30 - disabled backtrace

[compiler-errors]

r=me if ci is green

[Noratrieb]

@bors r=compiler-errors
ci is green

[bors]

📌 Commit 7098092 has been approved by compiler-errors

It is now in the queue for this repository.