cmd/install: add comment to deprecate --env.
cmd/install: add comment to deprecate --env.

We don't allow this on upgrade or reinstall so let's make `StdEnv`
essentially a private, internal API.

nandub/brew,EricFromCanada/brew,JCount/brew,Homebrew/brew,EricFromCanada/brew,vitorgalvao/brew,claui/brew,sjackman/homebrew,sjackman/homebrew,Homebrew/brew,claui/brew,JCount/brew,claui/brew,JCount/brew,MikeMcQuaid/brew,claui/brew,vitorgalvao/brew,sjackman/homebrew,MikeMcQuaid/brew,Homebrew/brew,EricFromCanada/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,Homebrew/brew,nandub/brew,JCount/brew,vitorgalvao/brew,sjackman/homebrew,nandub/brew,vitorgalvao/brew,nandub/brew,EricFromCanada/brew