feat(cli): add feature to disable downloads

[CathalMullan]

Resolves #3457, #3417

• Added 'no-downloads' feature to CLI.
• Switched to using rustc instead of rustup to determine what targets are installed.
• Lookup wasm-bindgen-cli from path when 'no-downloads' is enabled.
• Don't try and install targets using rustup when 'no-downloads' is enabled.
• Enable 'no-downloads' feature for Nix CI job.

Would have liked to have added tests, but mocking PATH is a little tricky to do safely across multiple tests.

Mainly need this for NixOS, but I suppose other environments may not want auto-downloading of tools either.

Also worth mentioning that the NixOS wiki now has a Dioxus page: https://wiki.nixos.org/wiki/Dioxus

[jkelleyrtp]

Thanks for the PR! I made a few changes

• Remove the need for a trait - just compressed the two implementations together
• Wired up no-downloads to be available regardless of the feature. You can now add this dynamically with NO_DOWNLOADS=1 or set the no_downloads cli setting at runtime.
• left the original structure in-tact

Whens checks pass I'll merge and release in 0.6.2 shortly