make kiln bake more deterministic

[crhntr]

This change set needs to be reconciled with #475. It should be a relatively simple rebase.

Given a commit sha and a clean git state
When `kiln Bake` builds a tile at two different times
Then the tile checksums are equal

This makes it easier to check if a tile has been tampered with and gives us some safety when building CI.

[cf-gitbot]

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