linux: provide access to stdout

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/app/app_nixos_test.go

@@ -2,6 +2,7 @@ package app_test
 
 import (
 	"fmt"
+	"io"
 	"io/fs"
 	"os/user"
 	"strconv"
@@ -579,6 +580,10 @@ func (s *stubNixOS) Exit(code int) {
 	panic("called exit on stub with code " + strconv.Itoa(code))
 }
 
+func (s *stubNixOS) Stdout() io.Writer {
+	panic("requested stdout")
+}
+
 func (s *stubNixOS) FshimPath() string {
 	return "/nix/store/00000000000000000000000000000000-fortify-0.0.10/bin/.fshim"
 }