Fix cert paths when not stored in GDK dir

What does this merge request do?

Fixes GitLab HTTP Router's Procfile options when NGINX SSL certificate paths are absolute paths.

Why?

I happen to specify absolute paths in my secondary Geo site's gdk.yml for the NGINX SSL certificate and key paths. I could copy them to the secondary Geo site's GDK directory, but--

I also notice that the NGINX paths are almost always wrapped in config.gdk_root.join(...), as in this line, so it seems safer to be more consistent.

The behavior of config.gdk_root.join(...) is that it doesn't modify the path if it is already absolute.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

I run two GDKs for Geo. I use HTTPS. I configure the secondary GDK to use the certs in the primary GDK's dir:

Click here to expand my secondary GDKs `gdk.yml`

---
gdk:
  runit_wait_secs: 40
  update_hooks:
    after:
    - make gitlab/git-checkout-auto-generated-files
    before:
    - gdk stop rails-background-jobs
    - pkill -lf 'sidekiq-cluster' || true
    - support/exec-cd gitlab bin/spring stop || true
    - rake gitlab:truncate_logs
geo:
  enabled: true
  secondary: true
hostname: gdk2.test
https:
  enabled: true
listen_address: 172.16.123.1
nginx:
  enabled: true
  http:
    port: 8081
  http2:
    enabled: true
  ssl:
    certificate: "/Users/mkozonogitlab/Developer/gdk/gdk.test+1.pem"
    key: "/Users/mkozonogitlab/Developer/gdk/gdk.test+1-key.pem"
port: 3444
postgresql:
  dir: "/Users/mkozonogitlab/Developer/gdk/postgresql"
sshd:
  enabled: true
  listen_port: 2223
  web_listen: 172.16.123.1:9123
vite:
  enabled: true
  hot_module_reloading: false
  port: 3809
webpack:
  enabled: false
workhorse:
  configured_port: 3334

Click here to expand my primary GDK's `gdk.yml` for reference

---
gdk:
  update_hooks:
    after:
    - make gitlab/git-checkout-auto-generated-files
    before:
    - gdk stop rails-background-jobs
    - pkill -lf 'sidekiq-cluster' || true
    - support/exec-cd gitlab bin/spring stop || true
    - rake gitlab:truncate_logs
geo:
  enabled: true
  secondary: false
gitlab:
  rails:
    allowed_hosts:
    - gdk2.test
hostname: gdk.test
https:
  enabled: true
listen_address: 172.16.123.1
nginx:
  enabled: true
  http2:
    enabled: true
  ssl:
    certificate: "/Users/mkozonogitlab/Developer/gdk/gdk.test+1.pem"
    key: "/Users/mkozonogitlab/Developer/gdk/gdk.test+1-key.pem"
port: 3443
sshd:
  enabled: false
vite:
  enabled: true
  hot_module_reloading: false
webpack:
  enabled: false

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• [-] Documentation added/updated, if needed.
• [-] Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.