Use compilation target instead of build machine target for MSRV checks

[foresterre]

I looked through the code and came to the conclusion that this crate seems to have an incorrect understanding of the way toolchains work.

• 1.63.0-thumbv7em-none-eabihf means that our build machine is a thumbv7em-none-eabihf. This is most likely always constant; a machine doesn't spontaneously change its type.
• The toolchain then has one or multiple installed targets. This is the compilation target machine, which is what the --target flag should actually control.

The current behavior, if being on an x86_64-unknown-linux-gnu machine and a --target thumbv7em-none-eabihf is specified and we want version 1.63.0, is:

• rustup install 1.63.0-thumbv7em-none-eabihf --profile minimal

What should happen instead is:

• rustup install 1.63.0 --profile minimal
• rustup target add --toolchain 1.63.0 thumbv7em-none-eabihf

Sadly, this makes this crate completely unusable for cross compilation crates (like many embedded crates).

Originally posted by @Finomnis in #587 (comment)