cmd/archive: output archive to stdout if - is passed as --archive-out

[roobre]

What?

This PR allows k6 to output archives to stdout when the special value - is used as the argument to --archive-out.

Why?

The only current way I know of to tell k6 to output an archive to stdout is to specify --archive-out=/dev/stdout. However, this relies on /dev existing, which in some fringe cases may not be the case, e.g. when running inside a chroot.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

https://github.com/grafana/sm-k6-archiver/pull/25

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.86%. Comparing base (23d8f91) to head (59ffa06).

❗ Current head 59ffa06 differs from pull request most recent head 2e5f64e

Please upload reports for the commit 2e5f64e to get more accurate results.

Files	Patch %	Lines
cmd/archive.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3797      +/-   ##
==========================================
- Coverage   71.87%   71.86%   -0.01%     
==========================================
  Files         291      291              
  Lines       21260    21263       +3     
==========================================
+ Hits        15280    15281       +1     
- Misses       4916     4918       +2     
  Partials     1064     1064              

Flag	Coverage Δ
ubuntu	71.81% <66.66%> (-0.01%)	⬇️
windows	71.71% <66.66%> (+0.01%)	⬆️

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