chunked: do not write cache file to RO store

[giuseppe]

if the layer is R/O, do not write a cache file.

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[giuseppe]

@rhatdan @mtrmac an easy one, but it solves an annoying warning when using an additional image store

[mtrmac]

Doesn’t this effectively completely stop looking for files in the read-only layers? It doesn’t stop writing the cache, it stops also creating it. And without an entry in c.layers, there can be no matches found in that layer.

*shrug* I don’t have a strong opinion on impact of that — avoiding cross-store linking might well be a good idea — but I’d like at least the comment in the code to be correct about the impact.

(And, ideally, the various implications of ALS/AIS, especially WRT cross-store sharing and dependencies, would be documented in detail somewhere, but I can see an argument that that’s not this PR.)

[rhatdan]

LGTM

[rhatdan]

/lgtm

[mtrmac]

I’d like at least the comment in the code to be correct about the impact.

#2079 .