Release the Pants PEX in --venv mode.

[jsirois]

This speeds things up:

$ hyperfine --warmup 2 './dist/pants.2.3.0.dev3+gitddc75f42..pex -V' '../pants/dist/pants.2.3.0.dev3+gitddc75f42..pex -V'
Benchmark #1: ./dist/pants.2.3.0.dev3+gitddc75f42..pex -V
  Time (mean ± σ):     636.7 ms ±  23.3 ms    [User: 441.7 ms, System: 35.9 ms]
  Range (min … max):   618.9 ms … 694.6 ms    10 runs

Benchmark #2: ../pants/dist/pants.2.3.0.dev3+gitddc75f42..pex -V
  Time (mean ± σ):      1.170 s ±  0.017 s    [User: 959.8 ms, System: 56.2 ms]
  Range (min … max):    1.155 s …  1.212 s    10 runs

Summary
  './dist/pants.2.3.0.dev3+gitddc75f42..pex -V' ran
    1.84 ± 0.07 times faster than '../pants/dist/pants.2.3.0.dev3+gitddc75f42..pex -V'

[ci skip-rust]

[jsirois]

I don't think anyone actually uses these, but...