screenShield: Don't inhibit suspend during initial setup

Normally, we inhibit suspend while locking the screen. But in the
session mode used for gnome-initial-setup locking is not supported, so
in that case this inhibit call is pointless and should be avoided.
Without this patch you get the following error when you suspend and
resume during initial setup:

JS ERROR: Error getting systemd inhibitor: Gio.IOErrorEnum:
GDBus.Error:org.freedesktop.login1.OperationInProgress: The operation
inhibition has been requested for is already running
_promisify/proto[asyncFunc]/</<@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:435:45

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1213

js/ui/screenShield.js

@@ -198,7 +198,7 @@ var ScreenShield = class {
         let lockEnabled = this._settings.get_boolean(LOCK_ENABLED_KEY);
         let lockLocked = this._lockSettings.get_boolean(DISABLE_LOCK_KEY);
         let inhibit = this._loginSession && this._loginSession.Active &&
-                       !this._isActive && lockEnabled && !lockLocked;
+                       !this._isActive && lockEnabled && !lockLocked && Main.sessionMode.unlockDialog;
         if (inhibit) {
             this._loginManager.inhibit(_("GNOME needs to lock the screen"),
                 inhibitor => {