ci: Try to avoid ffmpeg install failures (#4936)

A small portion of time -- maybe 10%, just enough for many CI workflow
runs to have a single job fail -- our attempt to `dnf install ffmpeg`
fails.

I thought maybe it was an intermittent, transient failure. But looping
to try multiple times with delays in between does not help. A job that
fails once either succeeds initially or fails over and over. I'm now
leaning toward the belief that rather than a transient failures, there
are just a subset of GHA runners that will always fail, and the
semi-random nature of our job failures is just random luck of the draw
in runner selection.

Anyway, so I'm not going to merge the try-in-a-loop approach. Instead,
this PR just makes ffmpeg an optional dependency, so the failures to get
the dependency won't fail the OIIO build and therefore the CI. It seems
like most of the time we really do get the dependency, but I'm out of
better ideas at the moment.

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

Author: lgritz

.github/workflows/ci.yml

@@ -277,7 +277,7 @@ 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: ${{ matrix.optional_deps || 'DCMTK;JXL;Libheif;Nuke;OpenCV;openjph;OpenVDB;Qt5;R3DSDK;'}}${{matrix.optional_deps_append}}
+      optional_deps: ${{ matrix.optional_deps || 'DCMTK;FFmpeg;JXL;Libheif;Nuke;OpenCV;openjph;OpenVDB;Qt5;R3DSDK;'}}${{matrix.optional_deps_append}}
     strategy:
       fail-fast: false
       matrix: