This change enables _FILE variants for all secrets in Healthchecks
configuration so they can be read from a file and not stored in
/nix/store.
In particular, it adds support for these secrets:
DB_PASSWORD, DISCORD_CLIENT_SECRET, EMAIL_HOST_PASSWORD,
LINENOTIFY_CLIENT_SECRET, MATRIX_ACCESS_TOKEN, PD_APP_ID,
PUSHBULLET_CLIENT_SECRET, PUSHOVER_API_TOKEN, S3_SECRET_KEY, SECRET_KEY,
SLACK_CLIENT_SECRET, TELEGRAM_TOKEN, TRELLO_APP_KEY, and TWILIO_AUTH.
