fix: cargo permission denied

[JimMoen]

...
sh: 1: exec: cargo: Permission denied
===> Hook for compile failed!
...

---

But:

$ dxcit -u root:root erlang bash
root@erlang:/emqx# ls /root/.cargo/bin/cargo
/root/.cargo/bin/cargo

[JimMoen]

cargo, rustc commands are linked to rustup.
so use rustup toolchain list to check the real actived toolchain