On a cache hit, the smoketest action should:
1) retrieves the base branch PR sources (at the time cache was built)
2) apply a diff patch, based on that original base SHA and the PR commit(s).
3) touches source to timestamp built - to ensure only next changes trigger re-makes.
4) switch/checkout current PR sha, which means only source differences are touched.

However, step 2) leaves the applied changes as working source differences, then 4) attempts a checkout merge.
Step 4) could cause merge conflicts. The build proceeds and fails.

It shouldn't be trying to merge, but fully switch in step 4).
One way is via git checkout --force, however that causes more than just the differences to be touched and therefore a lot more than is necessary will be rebuilt.

This can be avoided by committing the apply merge-patch, before checking out to the PR sha without force.