Add configurable GRAVITY_TMPDIR variable into setupVars

[kot0dama]

What does this PR aim to accomplish?:

This allows users to set up a Gravity updates temporary directory other than /tmp.
Indeed, some systems have a low amount of memory and some users might be reluctant to allocate too much memory to /tmp on these systems. You can look up the Internet for threads where people ask for a way to configure it.

How does this PR accomplish the above?:

I've added support for a GRAVITY_TMPDIR into gravity.sh, defaulting to /tmp if not configured, or if configured directory does not exist/is not a directory.

I've also added a default in the setup tool, I'm not sure that is the way to go though.

This way, users will be able to edit /etc/pihole/setupVars.conf and set/update GRAVITY_TMPDIR to the desired value.
This is probably unnecessary to add it as a step in the configurator though, but feel free to add it if it makes sense.

Link documentation PRs if any are needed to support this PR:

None

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[kot0dama]

Thanks for reviewing it, I think I've addressed all comments, let me know if there's any issue.
I've pushed another commit but then I can also squash them and force push.

Thank you!

[dschaper]

Thanks, can you squash and force please?

[kot0dama]

Thanks, can you squash and force please?

Done and merged development branch again ;)

[dschaper]

I don't know if this solves #5168 but it's very relevant to that.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-22-web-v5-19-and-core-v5-16-1-released/61999/1

[kot0dama]

Cool, that was fast !
Thank you !

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pihole-tmp-no-space-left-on-device/61797/5