WASIX: Running a package with --use leads to recursive filesystem merge

[theduke]

When running a package and injecting additional dependencies with --use, the WASIX FS falls back to a recursive merge, which is expensive and should not be necessary in this context.

wasmer run --use python/python wasmer/bash

2025-10-15T14:49:33.769490Z DEBUG ThreadId(01) merge: wasmer_wasix::fs: Falling back to a recursive copy to merge filesystems

[syrusakbary]

This is fixed by #5799 but to merge it, we need to remove webc v2 (and this is currently blocked until we migrate all old webcs to new ones)