useless self-update made

[rbtcollins]

When the rustup call is made, --no-self-update is not passed: as a result rustup does a self update, but this is useless as rustup will typically not be used again during the CI run.

When rustup update is needed, it will need to be done before installing rust - #170 .

So I suggest passing --no-self-update for any run installing rust components, and removing it if and only if calling rustup self update explicitly.

[striezel]

For anyone who needs a similar action but without the self-update, take a look at dtolnay/rust-toolchain.