Set process coding system to utf-8-auto

The default-process-coding-system on GNU/Linux is utf-8-unix (no BOM).
This results in an json parsing error when json-read tries to parse
the BOM on the first line as JSON:

(wrong-type-argument hash-table-p "Error: F# completion process produced malformed JSON")

byte order mark:
efbb bf7b 224b 696e 6422 3a22 494e 464f  ...{"Kind":"INFO