feat(installer): always check for cargo install first

[Pulko]

Hey 👋 This is my very first contribution that I dared 👉👈

Description of change

Closes #1603

• _install_cargo() function is checking if cargo and rustup are installed in the mid of _install_unsupported() whereas it could have been checked earlier to propose better ways of installation. I hope this would make sense and help distinguishing flows better

How has this been tested? (if applicable)

• checked for syntax mistakes
• ran through chat gpt lol
• ran locally and updated cargo-shuttle

[jonaro00]

Thanks for the PR! Looks like a good start.

[oddgrd]

Thanks for this @Pulko!