services: hpcguix-web: Set SSL_CERT_DIR.

Previously Git pulls over HTTPS would fail with:

  guix/git.scm:132:7: In procedure update-cached-checkout:
  Throw to key `git-error' with args `(#<<git-error> code: -17 message: "the SSL certificate is invalid" class: 16>)'.

* gnu/services/web.scm (hpcguix-web-shepherd-service): Pass
"SSL_CERT_DIR=/etc/ssl/certs".
* doc/guix.texi (Web Services): Mention certificates.