Check-only Clippy & Fmt for CI/CD

bjorn3:

The docker images don't have rustup at all, so there is no $RUSTUP_HOME.

I mean these rust images, which are labelled as official. Running a Shell via podman on rust:1.95.0-slim, it has obviously been installed by rustup:

# echo $RUSTUP_HOME $CARGO_HOME
/usr/local/rustup /usr/local/cargo
# ls -l $RUSTUP_HOME/toolchains/1.95.0-aarch64-unknown-linux-gnu/bin $CARGO_HOME/bin
/usr/local/cargo/bin:
total 19048
lrwxrwxrwx 1 root root        6 Apr 16 23:59 cargo -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 cargo-clippy -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 cargo-fmt -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 cargo-miri -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 clippy-driver -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rls -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rust-analyzer -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rust-gdb -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rust-gdbgui -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rust-lldb -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rustc -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rustdoc -> rustup
lrwxrwxrwx 1 root root        6 Apr 16 23:59 rustfmt -> rustup
-rwxrwxrwx 1 root root 19502880 Apr 16 23:59 rustup

/usr/local/rustup/toolchains/1.95.0-aarch64-unknown-linux-gnu/bin:
total 58296
-rwxrwxrwx 1 root root 40615904 Apr 16 23:59 cargo
-rwxrwxrwx 1 root root      980 Apr 16 23:59 rust-gdb
-rwxrwxrwx 1 root root     2160 Apr 16 23:59 rust-gdbgui
-rwxrwxrwx 1 root root     1060 Apr 16 23:59 rust-lldb
-rwxrwxrwx 1 root root  2333568 Apr 16 23:59 rustc
-rwxrwxrwx 1 root root 16728448 Apr 16 23:59 rustdoc

For the rest, thanks for your clarifications!