Revise eval system approach

[blaggacao]

tui/src/flake/run.go

Lines 105 to 116 in 3096bad

	if c.System == currentSystem {
	return nil, fmt.Errorf("set the --for flag to a different system than the current one ('%s')", currentSystem)
	}
	if c.System != "" {
	// if system is set, the impure flag provides a "hack" so that we
	// can transport this information to the action evaluation without
	// incurring in a prohibitively complex (m*n) data structure in
	// which we would have to account for _all_ combinations of current
	// and build system
	return []string{"--impure", c.renderFragmentFor(c.System)}, nil
	}

Revise these lines after NixOS/nix#4093 is merged and stabilized