do not rely on long deprecated FileOnMasterKeyStoreSource

[jtnord]

Adapt to jenkinsci/credentials-plugin#540 by not using code about to be removed

Testing done

mvn clean test -Dtest=CertificateMultiBindingTest#basics

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

Incomplete—this PR removed FileOnMasterKeyStoreSource from line 100, but there is still a usage 50 lines down.

[jtnord]

Incomplete—this PR removed FileOnMasterKeyStoreSource from line 100, but there is still a usage 50 lines down.

#312