manpage: add environment variables section (firmware, decoders path)

manpage: add environment variables section (firmware, decoders path)

Add the SIGROK_FIRMWARE_DIR, SIGROK_FIRMWARE_PATH, SIGROKDECODE_DIR, and
SIGROKDECODE_PATH environment variables to the sigrok-cli(1) manpage.

Strictly speaking these are the libraries' environment variables, when
libsigrok looks up device firmware, and libsigrokdecode searches for
Python scripts. But the libraries don't have individual manpages. So
let's put them here for better perception, and to have them _somewhere_
and not lose this information.