fix hardcoded temp directory for latex
Created by: avysk
<latex>...</latex>
block is converted to empty (size zero) svg on Mac OS X.
The reason: on Mac OS X temporary files are created not in /tmp but in another place (something along the lines of "/var/folders/^C/2b753l4s6sn3nckgqnf7v9zw0000gn/T"); dvisvgm is expecting to find generated .dvi there, but latex is instructed to output it to /tmp.