env: add analogue of `os.Getenv` and `os.Setenv`

[kamilsk]

	vars := env.Environment{
		env.Must("GIT_REMOTE", "git@github.com:octomation/maintainer.git"),
		env.Must("GITHUB_TOKEN", "secret"),
	}
	vars.Get("GIT_REMOTE")
	vars.Set("GIT_REMOTE", "new value")