jenkinsfile: Fix index variable definition so different runs of testNixGo don't write over each other.
jenkinsfile: Fix index variable definition so different runs of testNixGo don't write over each other.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client