Modify build-image to use global variable for image version

[samfreund]

Description

I added a global variable under settings, this variable controls the version of the image that is pulled down from https://github.com/PhotonVision/photon-image-modifier.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2025.3.2
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added

[samfreund]

This overlaps with #2110, if that gets merged first this will need modification to follow.

[mcm001]

This forces us to put all the images on the same pin. Do we want that? I assume yes?

[samfreund]

This forces us to put all the images on the same pin. Do we want that? I assume yes?

Ye, that's kinda the point. If there's an image in a release that's bad, we should cut a new one. If there's a new image we want to use, we should also cut a new one.

[samfreund]

@mcm001 so we can't reference variables on PRs from outside the repo. That's fine, cause we only run these jobs inside the repo. I believe the current config ought to work, but we can also create a branch inside the repo to test if we want.