Make DatalogManager writable check consistent between languages

[rzblue]

From wpilibsuite/SystemcoreTesting#155:

However, even after creating a symlink /u -> U my robot code still doesn't write to USB. It looks like C++ and Java will have slightly different behaviour here:

• It looks like C++ specifically checks that /u is world writable,
• whereas Java only checks that /u is writable.

[PeterJohnson]

std::filesystem doesn't have an equivalent operation to Java's isWritable, which uses access(2) with W_OK to check writability. So it looks like we'll need to call access(2) directly.

[auscompgeek]

Would making Java check the world-writable bit instead also be reasonable? Thinking aloud since the robot code runs as root on Systemcore currently, we probably want to avoid writing to a dir that'll be shadowed by a mount.

[PeterJohnson]

Robot code runs as the systemcore user in the most recent image version.

[auscompgeek]

Should teams actually be running the most recent Systemcore image? wpilibsuite/SystemcoreTesting#176 (comment) suggests teams shouldn't be.

[PeterJohnson]

Yes I know, but my broader point is we should be fixing this for the long term (when the user program is not root).