targets tries to avoid running two concurrent instances
of tar_make() on the same pipeline writing to the same data store.
Sometimes it generates false positives (meaning tar_make() throws
this error even though there is only one instance of the pipeline
running.) If there is a false positive, tar_unblock_process()
gets the pipeline unstuck by removing the _targets/meta/process file.
This allows the next call to tar_make() to resume.
Usage
tar_unblock_process(store = targets::tar_config_get("store"))