Ensure execution policy does not contain special chars

Signed-off-by: Erwan Leroy <herronelou@gmail.com>

src/rezplugins/shell/_utils/powershell_base.py

@@ -203,8 +203,9 @@ def _record_shell(ex, files, bind_rez=True, print_msg=False):
         cmd += ["-NoLogo"]
 
         # Powershell execution policy overrides
+        # Prevent injections/mistakes by ensuring policy value only contains letters.
         execution_policy = self.settings.execution_policy
-        if execution_policy:
+        if execution_policy and execution_policy.isalpha():
             cmd += ["-ExecutionPolicy", execution_policy]
 
         # Generic form of sourcing that works in powershell and pwsh