LocalCas*Instance: Do not attempt to parse paths as Directory protobuf
Description
Separate path strings and serialized Directory protobuf messages in UploadAndStore(). Valid path strings may be parsed as valid serialized protobuf messages, resulting in upload failure.
Validation
The code is exercised by existing test cases. Validation of the fix for the previously failing scenario is possible with the BuildStream BuildGrid integration test, see issue link below. The test now succeeds locally.