Avoid using `NSLock.withLock` because Linux doesn't support it before Swift 6.0

[d-ronnqvist]

Bug/issue #, if applicable: https://github.com/swiftlang/swift-docc-plugin/issues/92

Summary

This fixes a build error on Linux before Swift 6.0.

Dependencies

None

Testing

Build on Linux with Swift 5.10

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• ~[ ] Added tests~
• [x] Ran the ./bin/test script and it succeeded
• ~[ ] Updated documentation if necessary~

[d-ronnqvist]

@swift-ci please test

[hassila]

Thanks for quick turnaround!