rhg: set the expected temp file permissions (0o666 minus umask) This continues the theme of a48c688d3e80, and fixes the bug #6375, which was causing some problems for us, where a non-group-readable file can't be copied, which breaks some tools that copy the repo. This affects both the `checkexec` file and the temporary file we use for filesystem time measurement, since either of these files remaining on disk can cause this problem, and the 0666 permissions are just the better default here.