This commit updates `acme/http.go` to allow customizing the
`*x509.CertPool` used by the `HTTPClient` by specifying the filepath of
a custom CA certificate via the `CA_CERTIFICATE` environment variable.
This allows developers to easily trust a non-standard CA when
interacting with an ACME test server (e.g. Pebble):
```
CA_CERTIFICATE=~/go/src/github.com/letsencrypt/pebble/test/certs/pebble.minica.pem \
lego \
--server https://localhost:14000/dir \
--email foo@bar.com \
-d example.com \
run
```
