[ci] Error messages from missing `target/tests/target` directory

[joshlf]

@sl4m discovered this issue while working on #1276:

Note: I didn't see this as a GitHub issue, so wanted to flag the target clean up errors in this test run.

Originally, I thought I introduced these with the code change, but the errors appear in the official test runs as well. Example. It appears to be an issue with the action Swatinem/rust-cache, but haven't look into it further.