Ref T13209. See some discussion in T13209#241713.
There's a bug here where 0644 (a numeric literal written in octal) should be '0644' (a string literal). This caused the TempFile to fail to unlink() on Windows.
But with ArcanistWorkingCopyPath we don't have to write an actual file to disk at all, so just don't. Maybe we'll start doing this some of the time later on (e.g., to make it easier to test third-party linters which do not have a "read from stdin" mode), but ideally we shouldn't need to actually write to disk in order to test that linters work, at least in most cases.
Also fix the octal bug itself.