feat(zsh): check if `.cargo` exist before loading it

Signed-off-by: Seongmin Lee <[e-mail]>

zsh/.zprofile

···
 eval "$(/opt/homebrew/bin/brew shellenv)"
 
 # Rust
-. "$HOME/.cargo/env"
+if [ -f "$HOME/.cargo/env" ]; then
+  . "$HOME/.cargo/env"
+fi
 
 # bob-nvim
 export PATH=$HOME/.local/share/bob/nvim-bin:$PATH