OM2: generate tokenSource differently when running standalone/mmqueue locally with go run

[peterzhongyi]

Currently omclient.go doesn't generate a token source when running standalone/mmqueue locally with go run:

open-match-ecosystem/v2/internal/omclient/omclient.go

Line 379 in ce40d8f

if os.Getenv("K_REVISION") != "" { // Running on Cloud Run, get GCP SA token

We can generate a token source with google.DefaultTokenSource as suggested in googleapis/google-api-go-client#873 (comment) when users use go run to run the mmqueue.