T6525: Add default dir for ext-scripts without absolute path (backport #3827)

[mergify]

Change Summary

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6525

Related PR(s)

Component(s) name

Proposed changes

How to test

touch /config/user-data/test.sh
set service snmp script-extensions extension-name tst script test.sh 
commit

Smoketest result

vyos@vyos:~$ python3 /usr/libexec/vyos/tests/smoke/cli/test_service_snmp.py 
test_snmp_basic (__main__.TestSNMPService.test_snmp_basic) ... ok
test_snmp_script_extensions (__main__.TestSNMPService.test_snmp_script_extensions) ... ok
test_snmpv3_md5 (__main__.TestSNMPService.test_snmpv3_md5) ... 
service_snmp: ConfigError('Group membership required for user "vyos"!')


service_snmp: ConfigError('Must configure "view" for group "default_group"!')

ok
test_snmpv3_sha (__main__.TestSNMPService.test_snmpv3_sha) ... 
service_snmp: ConfigError('Must configure "view" for group "default"!')

ok
test_snmpv3_view_exclude (__main__.TestSNMPService.test_snmpv3_view_exclude) ... ok

----------------------------------------------------------------------
Ran 5 tests in 24.846s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3827 done by [Mergify](https://mergify.com).

[github-actions]

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