Enhancement: use capability-based runner detection instead of RUNNER_ENVIRONMENT

[mfn]

Related discussion: #1056

Description

Third-party runner providers (Depot, Blacksmith, Namespace, BuildJet) mirror GitHub's official runner images but set RUNNER_ENVIRONMENT=self-hosted.

The existing detection treats this as a bare self-hosted runner, routing PHP installation through the slower php-builder path (~>1min) instead of the fast pre-built php-ubuntu binaries (few seconds or at least <30s).

Replace the RUNNER_ENVIRONMENT check with capability-based detection: if both ImageOS and ImageVersion are set (and not ACT/container), classify as github.

This restores the pre-df8d1233 approach of trusting image markers over hosting metadata. Also replace the fail-fast guard with an informational log for third-party runner visibility.

In case this PR edits any scripts:

• I have checked the edited scripts for syntax.

• I have tested the changes in an integration test (If yes, provide workflow YAML and link).

  I can't provide a public link, I tested this in a private org owned repo with depot-hosted runners (and also with github runners, making sure both directions worked)
  Before:
  
  After:
  

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6ab9eec) to head (4f1fcab).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            8         8           
  Lines          853       853           
  Branches       305       305           
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  Hits           853       853           

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[shivammathur]

@mfn

The cached builds for GitHub that we have are only tested on GitHub runners and this change leads to people getting those builds on self-hosted runners.

I had initially based on the self-hosted check same as this PR just using $ImageOS and $ImageVersion, but that led to a lot of cases where these cached builds for GitHub runners were installed on self-hosted runners leading to broken setup and errors.

While this might work for certain commercial runners that have the same libraries as GitHub images, it breaks a lot of cases and increases the maintenance burden on my end.

[mfn]

Ok got it, I thought so; what a pity.

I figure this would require an opt-in from third party runner companies to declare somehow "compatible with github runner caches for setup-php", which is unlikely.

Welp. I'm out of ideas. Appreciated the summary 🙇🏼