fix: tilde expansion doesn’t working in quote

If nvm_custom_path is not set and nvm_type is not system, "~/nvm" is used as default nvm_path.
Then the next command is executed to load the nvm feature in nvm-exec.sh
```sh
source "~/nvm/nvm.sh"
```

However, the tilde expansion of posix shell does not work in a quoted string.
Using $HOME instead of ~ just works because variables are interpolated in a double quoted string.

Signed-off-by: Udagawa Kaito <umireon@gmail.com>