Fix a bug with --no-color and wrap Go's std logger with logrus

[na--]

As noted in #711, for some reason Go's standard library sometimes logs errors and warnings instead of returning them... But since we exclusively use github.com/sirupsen/logrus for logging in k6, those messages looked very different and didn't obey the --logformat configuration...

While fixing this, I noticed the (probably copy-paste) error stdout.Writer = colorable.NewNonColorable(os.Stderr) and fixed that as well 😅

[codecov-io]

Codecov Report

Merging #712 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #712      +/-   ##
==========================================
- Coverage   64.38%   64.37%   -0.01%     
==========================================
  Files         101      101              
  Lines        8275     8276       +1     
==========================================
  Hits         5328     5328              
- Misses       2598     2599       +1     
  Partials      349      349

Impacted Files	Coverage Δ
cmd/root.go	26.66% <0%> (-0.61%)	⬇️

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