Set matplotlib MPLCONFIGDIR env var in production

[iwyrkore]

In production we get:

[WARNING] 2022-06-21T16:16:26.921Z Matplotlib created a temporary config/cache directory at /tmp/matplotlib-gmtk3leg because the default path (/home/sbx_user1051/.config/matplotlib) is not a writable directory; it is highly recommended to set the MPLCONFIGDIR environment variable to a writable directory, in particular to speed up the import of Matplotlib and to better support multiprocessing.

We should set MPLCONFIGDIR in production to something in /tmp