Ensure cargo_bin_dir exists during install

[infifox]

I ran into this issue when doing a fresh install:

❯ curl -sSL https://dioxus.dev/install.sh | bash
######################################################################## 100.0%
Archive:  /home/nixos/.dx/bin/dx.zip
  inflating: /home/nixos/.dx/bin/dx
cp: cannot create regular file '/home/nixos/.cargo/bin/dx': No such file or directory
error: Failed to copy dx to /home/nixos/.cargo/bin

If the ~/.cargo/bin dir doesn't exist yet, the script gets stuck trying to copy to it.

[jkelleyrtp]

Thanks for the PR! I think we should just not copy dx to .cargo/bin/ if .cargo doesn't exist. Not sure we want to create the .cargo directory ourselves.