fix(docs): look up open issues on main builds for Renovate docs

[jamietanna]

Changes

We're currently always skipping the detection of open bugs/features, so
even though 0 introduced authentication to GitHub, this didn't
actually lead to the lookup when running in CI.

If we're not running in Renovate's CI build (i.e. as part of the release
process) or the docs site's CI build (as part of post-release
operations) we should continue to skip the build, when running in CI.

This also makes it clearer with a WARN log line to explain why this is
being skipped.

This is a re-apply of #39801, making sure that the GITHUB_TOKEN is explicitly
passed as
extraEnv, otherwise we see i.e.

ERROR: Error getting query results
       "err": {
         "cmd": "/bin/sh -c gh issue list --json \"title,number,url,labels\" --search \"type:Bug\" --limit 1000",
         "stderr": "gh: To use GitHub CLI in automation, set the GH_TOKEN environment variable.\n",
         "stdout": "",
         "options": {
           "encoding": "utf-8",
           "env": ["CI", "HOME", "PATH", "LANG"],
           "maxBuffer": 10485760,
           "timeout": 900000
         },
         "exitCode": 4,
         "name": "ExecError",
         "message": "Command failed: gh issue list ...\ngh: To use GitHub CLI in automation, set the GH_TOKEN environment variable.\n",
         "stack": "ExecError: Command failed: gh issue list ...\ngh: To use GitHub CLI in automation, set the GH_TOKEN environment variable.\n\n    at ..."
       }

As we can see, the env isn't automagically picking up the
GITHUB_TOKEN, so we should explicitly pass it.

Context

Please select one of the below:

• This closes an existing Issue: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
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• Yes — substantive assistance (AI generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

Tested on #39814 (hacked around in the CI setup to make sure only docs builds run)

Example job: https://github.com/renovatebot/renovate/actions/runs/19973679938/job/57284443849

Then looking at the built docs, see modules/platform/gitea/index.md now has a few issues related to it

[jamietanna]

I'm happy us not merging this until Monday (when I'm back in the office and can fix issues, if they arise)