nixos/qemu-vm: Use disposable EROFS for store when writableStore = false

This avoids putting a large disk image in the store (and possibly
in a binary cache), while improving runtime performance.

Assuming you're running an SSD, and/or with plenty of cache (?)
it is feasible to preempt the virtualization overhead before
VM start, in single-digit seconds.

For some tests that perform many reads on the store, the improved
performance of EROFS is sufficient that not only the image creation
overhead is compensated for, but is actually faster.

Stats for nixosTests.gitlab:

Baseline without useNixStoreImage: >1000s

Baseline with useNixStoreImage without writableStore = false
ext4 image in store: 277 seconds
  + significant image build time and/or disk space

Disposable erofs image: 249 seconds _including_ image build time

Custom erofs overlay on 9p host store: 391 seconds; presumably
because the overlay still performs too many 9p accesses, or perhaps
some other overhead. This solution had no obvious performance
advantage, while requiring extra options to work, so it was
discarded.

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