ci: Temporary fix -- make libheif optional for Ubuntu

This will get us through the great Canonical attack of May 2026, where
it seems that installing it on these machines is unreliable.

Revert this after everything is fixed.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

.github/workflows/ci.yml

@@ -353,7 +353,7 @@ jobs:
       # Override required_deps to be 'all' and explicitly list as optional
       # only the ones we are intentionally not testing for those jobs.
       required_deps: ${{ matrix.required_deps || 'all' }}
-      optional_deps: ${{ matrix.optional_deps || 'CUDAToolkit;DCMTK;JXL;Nuke;OpenGL;openjph;OpenVDB;Ptex;pystring;Qt5;R3DSDK;' }}${{matrix.optional_deps_append}}
+      optional_deps: ${{ matrix.optional_deps || 'CUDAToolkit;DCMTK;JXL;Nuke;OpenGL;openjph;OpenVDB;Ptex;pystring;Qt5;R3DSDK;Libheif;' }}${{matrix.optional_deps_append}}
       build_local_deps: ${{ matrix.build_local_deps }}
     strategy:
       fail-fast: false