Parallel building

[SheatNoisette]

Instead of compiling sequentially the programs, attempt to build them in parallel to speed up the compilation.

$ time v run build.vsh --cpus=4
Using 4 parallel jobs
compiling [                   ... took 691ms .
compiling arch                ... took 757ms .
[...]
compiling whoami              ... took 697ms .
compiling yes                 ... took 673ms .
> Compiled:  72 tools in 15321ms. Already compiled and skipped: 0 . All folders: 109 .
v run build.vsh --cpus=4  58.04s user 8.02s system 410% cpu 16.079 total

$ time v run build.vsh --cpus=1
Using 1 parallel jobs
compiling [                   ... took 454ms .
compiling arch                ... took 547ms .
compiling whoami              ... took 490ms .
compiling yes                 ... took 495ms .
> Compiled:  72 tools in 38784ms. Already compiled and skipped: 0 . All folders: 109 .
v run build.vsh --cpus=1  39.42s user 5.62s system 113% cpu 39.544 total

[JalonSolov]

Needs v fmt on build.vsh

[SheatNoisette]

@JalonSolov Done

[JalonSolov]

The CI errors are unrelated to this change.