Update andstor/file-existence-action in .github/workflows/build_docker_images.yml

[JC-wk]

Describe the bug
Build images observer warning
Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: andstor/file-existence-action@076e007. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Fixed in 3.1.0 https://github.com/andstor/file-existence-action/releases/tag/v3.1.0

Steps to reproduce

1. View warnings from github build output

Azure TRE release version (e.g. v0.14.0 or main):
main
Deployed Azure TRE components - click the (i) in the UI:
latest

[CodeVishal-17]

Hi! I'd like to work on this issue.

I'll update the GitHub Action to use the fixed version of andstor/file-existence-action, Thanks!

[JC-wk]

go ahead, should be straight forward