Investigate why InstanceCredentialProvider::cache is flagged as dead code

[alamb]

Describe the bug
In #5883 a new version of Clippy flagged InstanceCredentialProvider::cache as not used

To get CI clean again I disabled the clippy lint, but @Xuanwo rightly suggests we look into the root cause: https://github.com/apache/arrow-rs/pull/5883/files#r1638538556

To Reproduce

Expected behavior
TLDR we should figure out why this was flagged as not used, and either fix the underlying issue or decide that it is not a problem

Additional context

[Xuanwo]

Let me do this.