Make tests does not compile with indented toolchain 1.53.0

Problem

The commmand

make tests

leads to the output:

cargo +1.53.0  test
error: failed to download `plotters v0.3.2`

Caused by:
  unable to get packages from source

Caused by:
  failed to parse manifest at `/home/nice_graphic/.cargo/registry/src/github.com-1ecc6299db9ec823/plotters-0.3.2/Cargo.toml`

Caused by:
  feature `edition2021` is required

  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["edition2021"]` to enable this feature
make: *** [Makefile:65: tests] Error 101

When I however use stable or nightly as value for the TOOLCHAIN variable then no errors occur.

I noticed that, a similar error happens in the current pipeline for stage link-check as an example.

Output of

error: failed to compile `mdbook-linkcheck v0.7.7`, intermediate artifacts can be found at `/tmp/cargo-installrar8y5`
Caused by:
  failed to parse manifest at `/builds/bool_purist/ion/cargo/registry/src/github.com-1ecc6299db9ec823/clap_complete-3.2.5/Cargo.toml`
Caused by:
  feature `edition2021` is required
  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["edition2021"]` to enable this feature

My System.

I am using Linux/Fedora 37.

Output of

rustup show 

Default host: x86_64-unknown-linux-gnu
rustup home:  /home/<censored>/.rustup

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

stable-x86_64-unknown-linux-gnu
nightly-2022-03-18-x86_64-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu (default)
1.39.0-x86_64-unknown-linux-gnu
1.53.0-x86_64-unknown-linux-gnu
1.61.0-x86_64-unknown-linux-gnu

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

1.53.0-x86_64-unknown-linux-gnu (directory override for '/home/nice_graphic/Code/rust/gitlab/ion')
rustc 1.53.0 (53cb7b09b 2021-06-17)