repart: do not require /var/tmp if not used

repart: do not require /var/tmp if not used

If systemd-repart is running sufficiently early, /var/tmp might not be in place
yet. But if there is nothing to minimize, we won't even use it. Let's move the
check right before the first use.

systemd-repart[441]: Device '/' has no dm-crypt/dm-verity device, no need to look for…
systemd-repart[441]: Device /dev/sda opened and locked.
systemd-repart[441]: Sector size of device is 512 bytes. Using grain size of 4096.
systemd-repart[441]: Could not determine temporary directory: No such file or directory
systemd[1]: systemd-repart.service: Child 441 belongs to systemd-repart.service.
systemd[1]: systemd-repart.service: Main process exited, code=exited, status=1/FAILURE
systemd[1]: systemd-repart.service: Failed with result 'exit-code'.