Automatically switch rust-analyzer channel during rustup default

rust-analyzer

# assumption: you use `rustup component add rust-analyzer` to manage RA 
# you could check that by seeing whether is a rust-analyzer under `~/.rustup/toolchains/${your_toolchain}/bin`
# place this exectuable under dir like ~/.cargo/bin

#!/usr/bin/fish
eval (rustup which --toolchain (string split - (rustup default) | head -n 1) rust-analyzer) $argv