I thought the gem did this automatically if I wanted the default config,
but apparently not!
So this requires the SENTRY_DSN env-var to be set, and also in practice
requires SSL_CERT_DIR=/etc/ssl/certs if you don't have the certs
installed in guix as well.
Then we set the worker threads to 0 so it sends synchronously, since
otherwise we're about to quit and it'll just get enqueued and never
flushed.