Set version from runtime build info

[pablochacin]

Description

Use runtime information when setting the disruptor version that is used to identify the version of the agent image.
This is required to support building custom k6 images that use a specific version of the disruptor:

xk6 build --with github.com/grafana/xk6-disruptor=github.com/grafana/xk6-disruptor@vx.y.z 

Fixes #263

Checklist:

• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make test) and all tests pass.
• I have run relevant e2e test locally (make e2e-xxx for agent, disruptors, kubernetes or cluster related changes)
• Any dependent changes have been merged and published in downstream modules
  

[nadiamoe]

LGTM 👍🏻

Seems like windows build is failing though.

[pablochacin]

Seems like windows build is failing though.

Yes, it is because the windows build tool doesn't support versions that are not strictly numeric (e.g. v0.3.5) and fails with something like v0.3.5-rc1 which I used for testing.

[pablochacin]

@roobre I did some more testing and realized the solution I had proposed did not really fix the issue. So I reimplemented the logic for getting the version at runtime based on how the k6 version command does it.

[nadiamoe]

Interesting, so I suppose debug.ReadBuildInfo before was reading the k6 core version rather than the disruptor version, right?

lgtm