xk6: improve test robustness

[stefanb]

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Test often starts failing as soon as there is some new vulnerability found in one of the dependancies.

Ref:

• go 1.26.0, go@1.25 1.25.7 (new formula) #258912 (failing as a dependancy in most RC and release builds)
• govulncheck: revision bump (go 1.26) #266923 (failing as a dependancy, disabled dependancy builds with PR lable)
• Fix CVEs in dependencies grafana/xk6#401 (one of many pre-release fixes, but not often enough to expect it to be vulnerability-free all the time)
• ...