grub2: Show output when run in systemd by default

xref coreos/rpm-ostree#5071

Hiding errors by default is painful. At least as of
recently in Fedora it looks like the command is nice
and quiet by default, I only see

```
Generating grub configuration file ...
Adding boot menu entry for UEFI Firmware Settings ...
done
```

src/libostree/ostree-bootloader-grub2.c

@@ -419,7 +419,15 @@ _ostree_bootloader_grub2_write_config (OstreeBootloader *bootloader, int bootver
   grub_argv[2] = gs_file_get_path_cached (new_config_path);
 
   GSpawnFlags grub_spawnflags = G_SPAWN_SEARCH_PATH;
-  if (!g_getenv ("OSTREE_DEBUG_GRUB2"))
+  const bool running_in_systemd = getenv ("INVOCATION_ID") != NULL;
+  const bool debug_grub2 = g_getenv ("OSTREE_DEBUG_GRUB2");
+  /* If we're running in systemd (as part of `ostree-finalize-staged.service`)
+   * then we do want to gather output from the binary so that if something fails
+   * we can debug it.
+   *
+   * We also have an opt-in variable to display errors.
+   */
+  if (!(running_in_systemd || debug_grub2))
     grub_spawnflags |= G_SPAWN_STDOUT_TO_DEV_NULL | G_SPAWN_STDERR_TO_DEV_NULL;
   cdata.root = grub2_mkconfig_chroot;
   g_autofree char *bootversion_str = g_strdup_printf ("%u", (guint)bootversion);