support stale-cache restoring in AJ export CI action

[TheAfroOfDoom]

a follow-up to #81

the most common case of the AJ export cache becoming outdated will be when we update a relatively small amount of .ajblueprint files.

currently, the AJ exporter logic in CI (under the datapack/test job) will have to re-export every .ajblueprint, even if only a few actually changed

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we can speed up this process significantly by utilizing our last_exported_hashes.json logic and the cache action's restore-keys field:

• https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/caching-dependencies-to-speed-up-workflows#matching-a-cache-key
• https://github.com/actions/cache/blob/main/restore/README.md#inputs

together, these allow us to perform a partial restore using an old cache (e.g. one where most of the AJ export files are still up-to-date).

the auto-exporter already reads last_exported_hashes.json and automatically determines what .ajblueprints it actually needs to re-export. it skips over up-to-date export files. this would decrease the auto-exporter time down from a few minutes to a few seconds (depending on how many .ajblueprint files were updated in a commit)

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TODO of this ticket is to:

• add last_exported_hashes.json to the AJ export cache paths in our setup-animated-java-exports CI action
• add a restore-keys field (without the hashFiles... suffix) to the actions/cache/restore parameters so it can partial restore old AJ export caches of the same Blockbench/Animated Java version
• test the AJ export process speeds up when it needs to re-export only a few changed .ajblueprint files