Fix ffmpeg install

[subdavis]

• fixes [BUG] ppa ffmpeg savoury1/ffmpeg4 broken #1030
• fixes another bug introduced by Fix build error for worker #990 that broke ldc dev girder_worker_default
• fixes issue where method sig changes aren't picked up in dev mode.

Tested with tox integration tests.

[marySalvi]

Tested with a fresh build and works as expected.

[BryonLewis]

did a no-cache build with main to confirm the issue followed by one with this branch and it built properly.
Then did a series of uploads to the server that would require transcoding (image files, video files including AV1 video). Looks like all is good with this.

[BryonLewis]

Pulled built and tested again. Specifically tested worker container reloading. Modified apipeline task to add some additional logging, ran ldc dev up girder_worker_pipelines and properly verified that it reloaded the container and displayed the new logging info. The existing stuff also seemed to work such as ffmpeg conversion and AV1 support.