Unset TMPDIR before calling tempnam in tempname

[ivan-timokhin]

tempnam consults the TMPDIR environment variable before its argument (see man page), so if it is set the function will suddenly and quietly ignore the parent argument.

[KristofferC]

Ref #34726

[ivan-timokhin]

Ref #34726

Ugh. Fair enough. The problem is that the current conclusion for this case (C function depending on the environment) in the linked thread is "don't do that, use the library's configuration API", and the whole issue here is that tmpnam consults TMPDIR before its explicit parameters. So I'm not sure what would a better fix look like (aside from not using tmpnam at all—#22949).

If this approach is deemed unacceptable, I have no objections to closing this PR; I'll open an issue instead.

[ivan-timokhin]

I have thought about this some more, and convinced myself this is not going to work—as written, tempname would break any concurrent calls to tempdir, which is probably worse than the original problem. Mea culpa.