Handle PermissionError while listing available simulators (#820)

Author: std-max

vunit/sim_if/__init__.py

@@ -316,7 +316,10 @@ def isfile(file_name):
     Case insensitive Path.is_file()
     """
     fpath = Path(file_name)
-    if not fpath.is_file():
+    try:
+        if not fpath.is_file():
+            return False
+    except PermissionError:
         return False
 
     return str(fpath.name) in listdir(str(fpath.parent))