Use envconfig fork to abstract os package

[mstoykov]

Previous to this the envconfig package directly called os.LookupEnv.
With this it also takes a function that can replace this call giving us
the ability to not use for example map[string]string for the backing
instead of having to touch the global environment in tests.

[mstoykov]

I didn't use replace and instead changed the module path in the fork because:

1. I preferred it as it looks cleaner
2. but also i figured out later that otherwise this will break xk6 as replace clauses are only taken from the final module - if one of our dependency has raplace that won't have any effect, we will need to add the same replace in order for it to work.

Luckily, the changes for the most part are pretty small ... so that isn't really a problem.

The two other problems with envconfig seem to be more hairy so at least for now I don't plan on fixing them, but I am taking PRs :P

[mstoykov]

This is the actual envconfig change for which it was forked. It's backwards compatible on purpose, but we can change that if we decide in the future. The only other change is renaming the module.

[na--]

LGTM! One step closer... ☺️

[yorugac]

@mstoykov test-tip failed here: https://github.com/grafana/k6/runs/4938790631 ? That's not the flaky one it seems 😕

[mstoykov]

That's not the flaky one it seems

It is, it just fails really rarely.