Remove readonly from OID variables to allow multiple sourcing

Remove readonly declaration from OID_PERMIT_* variables to prevent
errors when certs.sh is sourced multiple times through different test
files (e.g., test-delegate-invalid-chain.sh sources test-delegate-onboarding.sh
which sources certs.sh).

Simpler solution than using source guards.

Fixes CI error:
'OID_PERMIT_REDIRECT: readonly variable'

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: runcom

test/utils/certs.sh

@@ -1,9 +1,9 @@
 #! /usr/bin/env bash
 
 # FDO delegate permission OIDs
-readonly OID_PERMIT_REDIRECT="1.3.6.1.4.1.45724.3.1.1"
-readonly OID_PERMIT_ONBOARD_NEW_CRED="1.3.6.1.4.1.45724.3.1.2"
-readonly OID_PERMIT_ONBOARD_REUSE_CRED="1.3.6.1.4.1.45724.3.1.3"
+OID_PERMIT_REDIRECT="1.3.6.1.4.1.45724.3.1.1"
+OID_PERMIT_ONBOARD_NEW_CRED="1.3.6.1.4.1.45724.3.1.2"
+OID_PERMIT_ONBOARD_REUSE_CRED="1.3.6.1.4.1.45724.3.1.3"
 
 generate_cert() {
   local key=$1