Use gettempdir, respecting tempdir enviroment variables

[aran3]

closes: #19206
Use tempfile.gettempdir() to get temp directory path, respecting TMPDIR and other standard environment variables - for creating the webserver cache, and also sqlite default location.
This is backward-compatible: if no special environment variable is set - it defaults to /tmp.

[boring-cyborg]

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[aran3]

Closing pull-request, as #19208 with the same fix by @potiuk