[9.3.0] Fix the persistent Merkle subtree cache never hitting for tree artifacts

[bazel-io]

Forked from #30786

[bazel-io]

Cherry-pick was attempted but there were merge conflicts in the following file(s). Please resolve manually.

src/test/java/com/google/devtools/build/lib/remote/merkletree/BUILD
src/test/java/com/google/devtools/build/lib/remote/merkletree/MerkleTreeComputerTest.java

cc: @bazelbuild/triage