Attempt to fix CI

[rukai]

This PR fixes a tricky CI caching issue introduced by #1801

#1801 pulled in astonbitecode/j4rs#115 which fixes j4rs so that the build.rs script runs on the first build only, instead of on the first and second builds only. (it never ran on 3rd, 4th etc. builds)

However we are accidentally relying on this fact in our CI setup.
We build j4rs which sets up the target/debug/jassets directory.
The main branch CI run caches the target directory excluding jassets since its not a typical rust target file.
On the next PR branch run CI restores target from the main branch cache such that j4rs is fully compiled (no build.rs to run) but the jassets directory is missing.
Previously, since the build.rs would rerun on the 2nd rerun the jassets directory would be repopulated.
But that is no longer the case, and so the tests explode since jassets is missing.

To fix this issue, this PR adds the jassets directory to the rust-cache github actions config.
Its impossible to test this without merging since the main branch cache write only occurs after a PR is merged.
But it should fix it and if it doesnt fix the issue we'll just try something else.

[codspeed-hq]

CodSpeed Performance Report

Merging #1806 will not alter performance

_{Comparing rukai:ci_fix (480b0ac) with main (157f199)}

Summary

✅ 38 untouched benchmarks