Imath-specific CI build image now available and VFX Platform 2024/2025/2026

[jfpanisset]

Currently ci_workflow_vfx.yml uses the ci-openexr container image:

Imath/.github/workflows/ci_workflow_vfx.yml

Line 39 in d062b80

image: aswf/ci-openexr:${{ matrix.vfx-cy }}

There is now an Imath-specific ci-imath container image:

https://hub.docker.com/r/aswf/ci-imath

for 2024 / 2025 / 2026. In particular the ci-openexr image includes a build of imath in /usr/local/ which is not necessarily desirable to test a "clean" build of Imath.

VFX Platform 2023 is no longer being updated and doesn't provide a ci-imath image.

I would suggest updating ci_workflow_vfx.yml to use the ci-imath image and years 2024/2025/2026.