Fix failing fuzzing test
This test has been failing for a while with the error:
go-fuzz-build -libfuzzer -o fuzz_variable_mask.a ./helpers/trace
00:47
/builds/gitlab-org/gitlab-runner/helpers/trace/buffer_fuzz.go:33:19: cannot use masks (variable of type []string) as common.MaskOptions value in argument to buffer.SetMasked
typechecking of ./helpers/trace failed
Uploading artifacts for failed job
e.g. https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/3858886416
No one's noticed before because the job is allowed to fail.