env: does not work for positional arguments

[nf-brentsaner]

Given:

type Args struct {
	Foo string `env:"FOO" short:"f" long:"foo"`
	Bar PosArg `positional-args:"1"`
}

type PosArg struct {
	Pos string `env:"BAR" positional-arg-name:"POSITIONAL_ARG"`
}

the execution of BAR='a non-empty string' <program> will currently never populate Args.Bar.Pos.

This behavior (or... lack thereof, technically) occurs whether the env: tag is defined on Args.Bar, PosArg.Pos, or both.

I recommend changing the approach of how environment variables are processed:

1.) First, before go-flags' tags are processed, run it through e.g. https://pkg.go.dev/github.com/caarlos0/env/v11. While this adds a dependency, it has the benefit of offloading env var processing logic from this project (and ensures env vars are handed per *NIX conventions, see below) (the particular library I linked to is, I think, the most popular and well-maintained env-to-struct library for Golang). This should then also handle nested structs (such as positional args).

2.) Then perform normal go-flags' tag processing/reflection.

This not only would resolve this bug, but it is also backwards-compatible (the library I linked uses the same tag name) and would respect *NIX conventions for priority; first CLI flags/args/switches, then env vars, then configuration files. (This priority ordering arose as a convention because it allows for runtime-contextual options (CLI invocation) to apply over session-contextual options (env vars), both of which can override system-contextual/default options (config files).)