ci: unbreak optix jubs by locking down container (#2041)

In all branches, the OptiX test job started failing several days ago,
corresponding to an update of the aswf/ci-osl:2025-clang18.3 container.
Reverting to aswf/ci-osl:2025-clang18.2 passes again!

Let's merge this fix so that our CI passes, and separately iterate
to figure out what went wrong with the newer container.

Signed-off-by: Larry Gritz <[email protected]>

Author: lgritz

.github/workflows/ci.yml

@@ -665,7 +665,7 @@ jobs:
             # select the right runner type and whether tests are run.
             nametag: linux-optix-vfx2025
             runner: ${{ (github.repository_owner == 'AcademySoftwareFoundation' && 'ubuntu-20.04-gpu-t4-4c-16g-176h') || 'ubuntu-latest' }}
-            container: aswf/ci-osl:2025-clang18
+            container: aswf/ci-osl:2025-clang18.2
             cxx_std: 17
             python_ver: "3.11"
             pybind11_ver: v2.11.1