cargo-fmt does not work with rustup 1.24.0

[EverlastingBugstopper]

Problem
cargo fmt can no longer be run as of rustup 1.24.0

Steps

$ rustup self update
info: checking for self-updates
info: downloading self-update
  rustup updated - 1.24.0 (from 1.23.1)
$ rustup update
rustup update     
info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'
info: syncing channel updates for 'nightly-x86_64-unknown-linux-gnu'
info: latest update on 2021-04-27, rust version 1.53.0-nightly (968425893 2021-04-26)
info: skipping nightly which is missing installed component 'rustfmt'
info: syncing channel updates for 'nightly-2021-04-26-x86_64-unknown-linux-gnu'
info: latest update on 2021-04-26, rust version 1.53.0-nightly (3709ae324 2021-04-25)
info: skipping nightly which is missing installed component 'rustfmt'
info: syncing channel updates for 'nightly-2021-04-25-x86_64-unknown-linux-gnu'
info: checking for self-updates

   stable-x86_64-unknown-linux-gnu unchanged - rustc 1.51.0 (2fd73fabe 2021-03-23)
  nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.53.0-nightly (42816d61e 2021-04-24)

info: cleaning up downloads & tmp directories
$ rustup component add rustfmt
info: component 'rustfmt' for target 'x86_64-unknown-linux-gnu' is up to date
$ cargo fmt 
error: unknown proxy name: 'cargo-fmt'; valid proxy names are 'rustc', 'rustdoc', 'cargo', 'rust-lldb', 'rust-gdb', 'rls', 'cargo-clippy', 'clippy-driver', 'cargo-miri'

Possible Solution(s)
This seems to work just fine on rustup 1.23.1, so we should likely be looking at recent changes

Notes

$ rustup --version
rustup --version           
rustup 1.24.0 (2f894d923 2021-04-24)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.51.0 (2fd73fabe 2021-03-23)`

$ rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /home/avery/.rustup

installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu (default)
nightly-x86_64-unknown-linux-gnu
1.47.0-x86_64-unknown-linux-gnu

installed targets for active toolchain
--------------------------------------

aarch64-apple-darwin
wasm32-unknown-unknown
x86_64-apple-darwin
x86_64-unknown-linux-gnu
x86_64-unknown-linux-musl

active toolchain
----------------

stable-x86_64-unknown-linux-gnu (directory override for '/home/avery/work/rover')
rustc 1.51.0 (2fd73fabe 2021-03-23)

[0xPoe]

It seems introduced by #2716.

[emilyalbini]

We reverted the production branch of Rustup to the known good version 1.23.1.