Move snapshotPathPrefix into a method

[jkutner]

This allows the value to be determined on the fly, which supports consumers that use Kaniko snaphot as a library and may need to change the value of config.KanikoDir

Description

The Snapshotter path prefix when invoking TakeSnapshotFS() is uses a local variable that can be overridden in tests. But this prevents the invoker of the TakeSnapshotFS() func from redefining the config.KanikoDir. By moving the snapshotPathPrefix into a getSnapshotPathPrefix() func, the value is resolved just-in-time, which allows the value to be configured again.

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