require_programs: run losetup with absolute path (MR 2475)

Use the absolute path for it, so it works in Debian too as we run it
without sudo here to just check if the argument is supported.

pmb/config/init.py

@@ -48,9 +48,10 @@ def require_programs() -> None:
     losetup_missing_json = False
 
     if "losetup" not in missing:
-        # Check if losetup supports the --json argument.
+        # 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(["losetup", "--json"], check=True)
+            pmb.helpers.run.user([pmb.config.required_programs["losetup"], "--json"], check=True)
         except RuntimeError:
             losetup_missing_json = True