Backfill MAJOR.MINOR Rustup channel manifests with a script this time

[carols10cents]

This is a replacement for #949

Now that rust-lang/rust#76107 has been merged,
new releases will also write their manifests to channel-rust-1.x.toml as
well as channel-rust-1.x.y.toml to enable rustup install 1.48 to get
the latest patch release in a minor release series.

This commit adds an idempotent script to copy manifests and their
signatures for the last patch release in every minor release series to
the corresponding minor manifest files so that past minor versions will
work with the rustup functionality too.

This script should only need to be run once, but should be safe to run
more than once.

It starts at 1.8 because we don't have manifests for 1.0-1.7, and it ends
with 1.47 because 1.48 will be the first stable release to write out the
1.x channel manifest.

r? @pietroalbini <3

[pietroalbini]

Ran the script locally! Thanks!

[pietroalbini]

Actually, ugh, the manifest were uploaded, but rustup doesn't like installing them:

pietro@january: ~/tmp$ rustup toolchain install 1.47
error: invalid toolchain name: '1.47'. Toolchain numbers tend to have three parts, e.g. 1.47.0

[Mark-Simulacrum]

I think @kinnison had indicated as such initially? It seems like that'll need a rustup release.

cc @XAMPPRocky as well since I think this might need to get dropped from relnotes for now in that case, I sort of doubt we'll have a rustup release in the next 3ish weeks

[kinnison]

I don't know off the top of my head if anyone has even filed an issue for supporting these channel files, but while I am hoping to get a release out soon I cannot guarantee to be done in 3 weeks, so it's probably best if we hold off announcing this for now.

[carols10cents]

o.O I totally missed the traffic here... thank you @pietroalbini for merging this and running it!

The issue for supporting these channel files is rust-lang/rustup#794 -- please don't announce until that is fixed and a new version of rustup is released! I was PURPOSEFULLY not doing the rustup part yet, so that we can have at least one release where the new channel files are being written by the release process (which will be 1.48). We can even wait for a point release to happen if we want to be really conservative about releasing this functionality.