Write file with channel to S3

[Mark-Simulacrum]

This will let rustup-toolchain-install-master gain support for installing stable artifacts, which is currently only possible when explicitly overriding the channel. That in turn will unblock letting Crater kick off a beta run as soon as both a new beta and a new stable artifact are ready, rather than waiting until the actual release.

[Mark-Simulacrum]

The goal is to replace the code here: https://github.com/kennytm/rustup-toolchain-install-master/blob/5dc0ab4060685445311ac7224d12cd60bd6df175/src/main.rs#L370 with a single fetch of the appropriate artifact.

@bors try

[rust-bors]

💔 Test for cf1d71c failed: CI. Failed jobs:

• try - dist-x86_64-linux (web logs, enhanced plaintext logs)

[Mark-Simulacrum]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: bcde5a8 (bcde5a8325a0846881ff81f5aeb7b2493caa8e77, parent: b53da9908134f77da3bb36e7009ac81f02ee48dc)

[rustbot]

r? @marcoieni

rustbot has assigned @marcoieni.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Mark-Simulacrum]

Try jobs:

• https://github.com/rust-lang/rust/actions/runs/20414608706 for nightly
• https://github.com/rust-lang/rust/actions/runs/20414617048/job/58656444417 for stable

Presuming those produce expected artifacts should be ready for review.

[rust-bors]

💔 Test for d9d71ef failed: CI. Failed jobs:

• try - dist-x86_64-linux (web logs, enhanced plaintext logs)

[Mark-Simulacrum]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 2a59135 (2a59135288568ce16db9c4f25263a2c29c8cce11, parent: a6525d5264da34f51ad48c178281d3c6323dbfcf)

[Kobzol]

https://ci-artifacts.rust-lang.org/rustc-builds/2a59135288568ce16db9c4f25263a2c29c8cce11/package-version looks good!

You can r=me.