HTTPProxy gRPC plaintext test flake

[skriss]

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https://github.com/projectcontour/contour/actions/runs/7544369941/job/20537699527?pr=6084

  	Error Trace:	/home/runner/work/contour/contour/test/e2e/httpproxy/grpc_test.go:122
  	            				/home/runner/go/pkg/mod/github.com/onsi/ginkgo/[email protected]/internal/node.go:463
  	            				/home/runner/go/pkg/mod/github.com/onsi/ginkgo/[email protected]/internal/suite.go:889
  	            				/opt/hostedtoolcache/go/1.21.6/x64/src/runtime/asm_amd64.s:1650
  	Error:      	Received unexpected error:
  	            	rpc error: code = PermissionDenied desc = 
  	Test:       	HTTPProxy gRPC tests with namespace: grpc-upstream-plaintext requests to a gRPC service configured with plaintext work as expected
  	Messages:   	gRPC error code 7