Directly call *env commands to avoid hook path duplication

[jmcantrell]

I noticed that any time I use *env latest install, any after_install hooks were being called twice.

Eventually, I found that this (would apply to any $COMMAND, but I'm using luaenv as an example) in combination with the fact that xxenv-latest is calling the install commands like $COMMAND install is causing the hook directories to be added twice. The first time, when *env latest ... is called by the user and then again when xxenv-latest calls the install command.

Since *env sets up PATH such that the subcommands can be used directly, it makes more sense to call them like $COMMAND-$SUBCOMMAND. This ensures the hook PATH is only altered once.

I went ahead and changed every call (not just install) to be consistent, but I don't know that it would affect others, since {before,after}_install seem to be the only hooks. There could be more in the future, and this would ensure all the commands have a clean environment.

Kind of related, but would you be open to me adding some unit testing in another PR?

[jmcantrell]

I should have created a different branch for the other nitpicky stuff. Would you prefer if I closed this and reworked it?

[momo-lab]

It's no problem as it is. However, please give me time to verify the PR.