[CI] Set BUILD_AGAINST_ALL_FFMPEG_FROM_S3 for torchcodec source build

torchcodec's setup.py requires this env var when building a wheel
to handle FFmpeg bundling. Without it, the build fails with a
ValueError about license confirmation.

Made-with: Cursor

Author: vmoens

.github/unittest/linux/scripts/run_all.sh

@@ -240,7 +240,7 @@ fi
 
 # install torchcodec (from source for nightly to match PyTorch ABI)
 if [[ "$RELEASE" == 0 ]]; then
-  uv_pip_install --no-build-isolation git+https://github.com/meta-pytorch/torchcodec.git
+  BUILD_AGAINST_ALL_FFMPEG_FROM_S3=1 uv_pip_install --no-build-isolation git+https://github.com/meta-pytorch/torchcodec.git
 else
   uv_pip_install torchcodec
 fi