Specifying --vus and --iterations prevents execution of browser checks

[roobre]

#829 introduced an image flavor that allows running browser checks. However, for them to be fully runnable, we need to get rid of --vus, --iterations, and --duration arguments: grafana/k6#3742

If specified, k6 will yield an error like the following:

ERRO[0000] Uncaught (in promise) GoError: browser not found in registry. make sure to set browser type option in scenario definition in order to use the browser module
	at github.com/grafana/xk6-browser/browser.syncMapBrowser.func7 (native)
	at default (file:///home/roobre/Devel/crocochrome/manualtest/testk6io.js:20:31(3))  executor=shared-iterations scenario=default

We currently specify two of those:

synthetic-monitoring-agent/internal/k6runner/k6runner.go

Lines 589 to 590 in 083d43b

	"--vus", "1",
	"--iterations", "1",

We should figure out a way to condition those arguments to the check being a browser check, or find another workaround.

Blocked by #928, which will inform the runner of the script type and allow it to not specify these options.

[peterschretlen]

This is blocking browser checks running in private probes.

[roobre]

Renamed the issue to make it a bit more discoverable.