Install a backend with nargo command

[kobyhallx]

As a Developer of Noir Porgrams
I would like to be able to install a backend
so that I can fulfil my development workflow with prove and verify nargo subcommands.

AC:

1. When nargo install is successfull backend binaries are present on user system and available 'globally'
2. Invocation of backend related commands (ie. prove) is able to find globally installed backend and utilise it for it's task
3. subsequent invocation of install set's currently installed backend as default

[TomAFrench]

I'm going to delay this until I've got most of the other commands working. There are a couple of annoyances with how the bb binary is distributed (AztecProtocol/aztec-packages#1519) so I'd like to get things working with the binary installed manually and then we can explore this once those issues are resolved.