If the test cannot find a temp directory by
checking environment variables, it will fall back
to using the current working directory as the temp
directory root.

This is what the test used to do as of commit
https://github.com/google/protobuf/commit/6de51caed52d798815954646b230c5aef3e4d2fc
and what was then changed by commit
https://github.com/google/protobuf/pull/3978/commits/792d098769d8e000d8d474c8ffd201d2eabc2134