Safeguard for charmbracelet/bubbletea#964 prevents using regular stdin

[cardil]

The following block prevents from using regular stdin with tui.Spinner and tui.Progress:

client-pkg/pkg/output/tui/bubbletea964.go

Lines 38 to 45 in ca2b56e

	if !st.Mode().IsRegular() {
	if st.Name() != os.DevNull {
	log.Println("WARN: non-regular file given as input: ",
	st.Name(), " (mode: ", st.Mode(),
	"). Using `nil` as input.")
	}
	return nil
	}

The following situation happens:

$ go run github.com/cardil/repro-kn-cli-bad-license@latest
2024/03/27 11:37:43 WARN: non-regular file given as input:  stdin  (mode:  Dcrw--w---- ). Using `nil` as input.

/kind bug