fix: has_component check

[eightfilms]

Closes #274

Adding an invalid component currently panicks, as reported by @camiinthisthang:

$ fuelup component add [email protected]
thread 'main' panicked at 'no entry found for key', src/toolchain.rs:190:28
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is caused by the has_component() check which was checking if a component existed in the currently active toolchain before trying to add it, by doing component[component]. We should use get() instead.