After installing a toolchain, default to that toolchain

[sdankel]

Rustup does this, and it saves the user from some confusion and having to type another command to switch.

[JoshuaBatty]

Hmmm I don't see that behaviour. Is this what you were doing?

 ~/Doc/r/fuel/fuelup  josh/install_and_default *2  rustup show active-toolchain                              ok  14:33:35
stable-aarch64-apple-darwin (default)

 ~/Doc/r/fuel/fuelup  josh/install_and_default *2  rustup toolchain install nightly                          ok  14:48:58
info: syncing channel updates for 'nightly-aarch64-apple-darwin'
info: latest update on 2024-06-28, rust version 1.81.0-nightly (9c3bc805d 2024-06-27)
info: downloading component 'llvm-tools'
 36.4 MiB /  36.4 MiB (100 %)   4.1 MiB/s in  8s ETA:  0s
info: downloading component 'rust-src'
info: downloading component 'rustc-dev'
 96.1 MiB /  96.1 MiB (100 %)   5.2 MiB/s in 23s ETA:  0s
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-docs'
info: downloading component 'rust-std'
 24.9 MiB /  24.9 MiB (100 %)  24.2 MiB/s in  1s ETA:  0s
info: downloading component 'rustc'
 52.3 MiB /  52.3 MiB (100 %)  22.7 MiB/s in  2s ETA:  0s
info: downloading component 'rustfmt'
info: removing previous version of component 'llvm-tools'
info: removing previous version of component 'rust-src'
info: removing previous version of component 'rustc-dev'
info: removing previous version of component 'cargo'
info: removing previous version of component 'clippy'
info: removing previous version of component 'rust-docs'
info: removing previous version of component 'rust-std'
info: removing previous version of component 'rustc'
info: removing previous version of component 'rustfmt'
info: installing component 'llvm-tools'
 36.4 MiB /  36.4 MiB (100 %)  15.5 MiB/s in  3s ETA:  0s
info: installing component 'rust-src'
info: installing component 'rustc-dev'
 96.1 MiB /  96.1 MiB (100 %)  15.2 MiB/s in  6s ETA:  0s
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-docs'
 15.8 MiB /  15.8 MiB (100 %)   4.3 MiB/s in  3s ETA:  0s
info: installing component 'rust-std'
 24.9 MiB /  24.9 MiB (100 %)  14.8 MiB/s in  1s ETA:  0s
info: installing component 'rustc'
 52.3 MiB /  52.3 MiB (100 %)  16.0 MiB/s in  3s ETA:  0s
info: installing component 'rustfmt'

  nightly-aarch64-apple-darwin updated - rustc 1.81.0-nightly (9c3bc805d 2024-06-27) (from rustc 1.80.0-nightly (9cdfe285c 2024-05-22))

info: checking for self-update

 ~/Doc/r/fuel/fuelup  josh/install_and_default *2  rustup show active-toolchain                       ok  1m 5s  14:50:14
stable-aarch64-apple-darwin (default)