Run tr with LC_ALL=C to avoid errors with invalid UTF-8 on Mac

With a locale like `en_US.UTF-8`, `tr` complains about invalid UTF-8
when running `cat /dev/urandom | tr ...`.

nix/services.nix

@@ -67,7 +67,7 @@
         fi
         set -x
         cp deployment/localsettings.template.py evap/localsettings.py
-        sed -i -e "s/\$SECRET_KEY/$(head /dev/urandom | tr -dc A-Za-z0-9 | head -c 32)/" evap/localsettings.py
+        sed -i -e "s/\$SECRET_KEY/$(head /dev/urandom | LC_ALL=C tr -dc A-Za-z0-9 | head -c 32)/" evap/localsettings.py
         git submodule update --init
         ./manage.py migrate --noinput
         ./manage.py collectstatic --noinput