Optionally use download_queue for brew install

[MikeMcQuaid]

Still WIP:

Performance testing on my M1 Max MacBook Pro (that we do make -j10 on):

Benchmark 1: HOMEBREW_DOWNLOAD_CONCURRENCY=40 brew install ack wget ffmpeg
  Time (mean ± σ):     107.526 s ±  3.498 s    [User: 31.709 s, System: 65.326 s]
  Range (min … max):   105.053 s … 110.000 s    2 runs
 
Benchmark 2: HOMEBREW_DOWNLOAD_CONCURRENCY=20 brew install ack wget ffmpeg
  Time (mean ± σ):     100.833 s ±  3.007 s    [User: 31.519 s, System: 62.292 s]
  Range (min … max):   98.706 s … 102.959 s    2 runs
 
Benchmark 3: HOMEBREW_DOWNLOAD_CONCURRENCY=10 brew install ack wget ffmpeg
  Time (mean ± σ):     103.043 s ±  1.297 s    [User: 31.360 s, System: 63.587 s]
  Range (min … max):   102.126 s … 103.960 s    2 runs
 
Benchmark 4: HOMEBREW_DOWNLOAD_CONCURRENCY=2 brew install ack wget ffmpeg
  Time (mean ± σ):     139.044 s ±  3.662 s    [User: 31.711 s, System: 64.411 s]
  Range (min … max):   136.454 s … 141.633 s    2 runs
 
Benchmark 5: brew install ack wget ffmpeg
  Time (mean ± σ):     162.942 s ±  0.163 s    [User: 32.008 s, System: 57.277 s]
  Range (min … max):   162.827 s … 163.057 s    2 runs
 
Summary
  HOMEBREW_DOWNLOAD_CONCURRENCY=20 brew install ack wget ffmpeg ran
    1.02 ± 0.03 times faster than HOMEBREW_DOWNLOAD_CONCURRENCY=10 brew install ack wget ffmpeg
    1.07 ± 0.05 times faster than HOMEBREW_DOWNLOAD_CONCURRENCY=40 brew install ack wget ffmpeg
    1.38 ± 0.05 times faster than HOMEBREW_DOWNLOAD_CONCURRENCY=2 brew install ack wget ffmpeg
    1.62 ± 0.05 times faster than brew install ack wget ffmpeg