Use files.gravity_tmp as temporary directory for the intermediate lists

[DL6ER]

What does this implement/fix?

See title. This PR depends on pi-hole/FTL#1796. This addition is necessary to preserve the ability to change the temporary directory. In v5.0, this was possible via setupVars.conf

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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[pralor-bot]

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

https://discourse.pi-hole.net/t/gravity-tmpdir-from-setupvars-conf/66667/2

[yubiuser]

I set the variable to a non existing path and it fails, however, the fallback should kick in

nanopi@nanopi:~$ pihole -g
  Gravity temporary directory does not exist or is not a writeable directory, falling back to /tmp. 
  [i] Creating new gravity database
Error: unable to open database "/etc/pihole/gravity.db
files.gravity_tmp = /tmp/test": unable to open database file
   [✗] Unable to create /etc/pihole/gravity.db
files.gravity_tmp = /tmp/test
   [✗] Error creating new gravity database. Please contact support.
   [✗] Unable to migrate to database. Please contact support.

[DL6ER]

@yubiuser This is actually an issue in FTL returning both files.gravity and files.gravity_tmp when requesting files.gravity the last commit in the corresponding FTL PR fixes this.