feat(ci): Trigger security scanner from release pipeline (#63280)

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Resolves
[REL-100](https://linear.app/sourcegraph/issue/REL-100/automate-posting-the-security-check-step)
by calling the security scanner as part of the release pipeline, during
the internal image creation time.

@WillDollman kindly dropped me some notes on how to use this, notably in
this PR I've removed `dryRun=true` given how this will only be run from
the release process. I'm also using a `tag` with no `images` listed.

I've added the token will sent me to 1Password under
`image-scanner-webhook` and added a check to ensure it exists when being
called.

### Will's Notes: 

```
curl --location 'https://incoming.sgdev.org/new-image-scan?images=sourcegraph%2Fgitserver%2Csourcegraph%2Ffrontend&tag=5.3.0&scanType=release&dev=true&dryRun=true' \
--header 'X-Special-Header: <key-shared-in-1password>'
```

> it’s not quite set up for releases yet, but you can play around -
results are sent to elastic which you don’t have access to, and there’s
no api to get the results

> to scan everything at a specific tag, remove the images parameter and
set tag to the image tag

> dryRun=true will ensure it doesn’t actually run a scan, but will still
return a json response - best to leave that enabled while you’re working
on it otherwise you’ll trigger lots of scans 😛
## Test plan

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Ran `sg release create --version=auto --pretend` to ensure the script
still works

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- Added security scanner to the release pipeline

release.yaml

@@ -73,6 +73,9 @@ internal:
             echo "Release created, see:"
             echo $body
           fi
+      - name: 'Trigger Security scan'
+        cmd: |
+          curl --location 'https://incoming.sgdev.org/new-image-scan?tag={{tag}}&scanType=release&dev=true' --header 'X-Special-Header: ${SCANNER_TOKEN}'
       - name: 'notifications'
         cmd: |
           set -eu