Keep the additive resolver reachable after the upstream checkout

[danielhanchen]

The fallback added in #71 never ran. The first real release attempt hit the three expected add/add conflicts and refused anyway:

CONFLICT (content): Merge conflict in src/llama-arch.cpp
CONFLICT (content): Merge conflict in src/llama-model.cpp
CONFLICT (content): Merge conflict in tools/mtmd/CMakeLists.txt
unslothai/llama.cpp#70 ... does not merge cleanly onto b10290

Resolve checks the upstream base tag out into this repo's own working tree, so by the time the merge runs, the tree is upstream's and scripts/unsloth/additive_merge.py is not in it. python3 scripts/unsloth/additive_merge.py failed on a missing file, and the if treated that as "could not resolve".

The preflight was unaffected, and that is why it passed: it clones upstream into a subdirectory and calls the script from the parent checkout, which stays intact.

Copy the resolver to $RUNNER_TEMP before the checkout and call it there.