fix(tekton e2e): Fix CONTAINER_REGISTRY name generation

[navidshaikh]

var E2E_BASE_NAME is no longer available

[knative-prow-robot]

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[navidshaikh]

-------------------------------
---- Running test pipeline ----
-------------------------------
---- Mon Oct 26 07:06:56 PDT 2020
-------------------------------
Running go test with args: -race -count=1 -timeout=30m -tags=tekton ./test/e2e
=== RUN   TestTektonPipeline
--- PASS: TestTektonPipeline (152.77s)
PASS
ok  	knative.dev/client/test/e2e	152.879s
DONE 1 tests in 220.755s

tests passed ref: logs
They were failing earlier with mentioned issue https://testgrid.knative.dev/client#tekton
cc: @mgencur

reverting the debug commit

[mgencur]

/lgtm

Thanks!