require_programs: losetup --json: set output=null (MR 2475)

We don't care about the output of "losetup --json" here, we just want to
make sure that the argument exists. If it does not exist, we already
give an error to the user.

Set the output to null so it does not clutter the log since it runs with
every pmbootstrap command now.

pmb/config/init.py

@@ -51,7 +51,9 @@ def require_programs() -> None:
         # Check if losetup supports the --json argument. Use the absolute path
         # here, so it works in Debian too without using sudo.
         try:
-            pmb.helpers.run.user([pmb.config.required_programs["losetup"], "--json"], check=True)
+            pmb.helpers.run.user(
+                [pmb.config.required_programs["losetup"], "--json"], check=True, output="null"
+            )
         except RuntimeError:
             losetup_missing_json = True