Only mount X509_USER_PROXY in containers if the files exist

Goes with LbEnv!138 (merged)

LbPlatformUtils/debug.py

@@ -227,7 +227,9 @@ def run_apptainer(results, cmd, silent=False, verbose=False):
         os.getcwd(),
         "--userns",
     ]
-    if "X509_USER_PROXY" in os.environ:
+    if "X509_USER_PROXY" in os.environ and os.path.isfile(
+        os.environ["X509_USER_PROXY"]
+    ):
         apptainer_cmd_base += ["--bind", os.environ["X509_USER_PROXY"]]
     apptainer_cmd_base += ["/cvmfs/cernvm-prod.cern.ch/cvm3"]
     return run_cmd(results, apptainer_cmd_base + cmd)