[CI] Set Python3_ROOT_DIR for torchcodec CMake build

CMake's FindPython3 can't locate Python include headers in uv's
standalone Python installation. Setting Python3_ROOT_DIR to the
base prefix lets CMake find the development files.

Made-with: Cursor

Author: vmoens

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

@@ -242,8 +242,10 @@ fi
 # install torchcodec (from source for nightly to match PyTorch ABI)
 if [[ "$RELEASE" == 0 ]]; then
   export BUILD_AGAINST_ALL_FFMPEG_FROM_S3=1
+  export Python3_ROOT_DIR="$(python -c 'import sys; print(sys.base_prefix)')"
   uv_pip_install --no-build-isolation git+https://github.com/meta-pytorch/torchcodec.git
   unset BUILD_AGAINST_ALL_FFMPEG_FROM_S3
+  unset Python3_ROOT_DIR
 else
   uv_pip_install torchcodec
 fi