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Summary:
Currently, macOS CI jobs install `pkg-config` and `wget` with `brew`.
This is problematic as brew takes a long time with auto-update, and disabling the auto-update is not an ideal solution.
Conda also distributes these packages, so switching to conda.

Example issues with brew installation.
https://app.circleci.com/pipelines/github/pytorch/audio/7825/workflows/53965bcf-6ddf-4e42-ad52-83fd1bbab717

This commit removes the use of `brew` by
1. Replacing the use of `wget` with `curl` (pre-installed in most distro)
2. Install `pkg-condig` from conda.
    Note: All the macOS jobs, including binary build jobs, uses conda. Using `pkg-config` from Conda makes it easy to discover the packages installed from conda. (like `ffmpeg` in https://github.com/pytorch/audio/issues/2122)
3. Add `pkg-config` to conda build-time dependency
4. Make sure that the availability of `pkg-config` is explicitly checked when `sox` is being configured. (otherwise, it will fai...