Catch timeouts when checking for GPG availability.

Fixes #428. Okay to do this because we will fail tests if GPG is unexpectedly
unavailable (#434).

Signed-off-by: Zachary Newman <zjn@chainguard.dev>

securesystemslib/gpg/constants.py

@@ -18,6 +18,7 @@
 import functools
 import logging
 import os
+import subprocess
 
 from securesystemslib import process
 
@@ -30,7 +31,7 @@ def is_available_gnupg(gnupg: str) -> bool:
   try:
     process.run(gpg_version_cmd, stdout=process.PIPE, stderr=process.PIPE)
     return True
-  except OSError:
+  except (OSError, subprocess.TimeoutExpired):
     return False