macOS github action failing

[CDrummond]

macOS builds currently disabled as github action fails:

  cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64-apple-darwin
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64_apple_darwin
  cargo:rerun-if-env-changed=HOST_PKG_CONFIG_SYSROOT_DIR
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR

  --- stderr
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: `PKG_CONFIG_ALLOW_SYSTEM_CFLAGS="1" PKG_CONFIG_ALLOW_SYSTEM_LIBS="1" "pkg-config" "--libs" "--cflags" "libavutil"` did not exit successfully: exit status: 1
  error: could not find system library 'libavutil' required by the 'ffmpeg-sys-next' crate

  --- stderr
  Package libavutil was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libavutil.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'libavutil' found
  ', /Users/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ffmpeg-sys-next-6.0.1/build.rs:715:14
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
Error: Process completed with exit code 101.