ci(test): fix unhandled buildkitd tags

[crazy-max]

Reported by @jedevc

This is a regression from #4048: https://github.com/moby/buildkit/actions/runs/6337159614/job/17211754423?pr=4288#step:8:1478

=== Failed
=== FAIL: client TestNydusIntegration/TestBuildExportNydusWithHybrid/worker=containerd (6.35s)
    client_nydus_test.go:79: 
        	Error Trace:	/src/client/client_nydus_test.go:79
        	            				/src/client/client_nydus_test.go:133
        	            				/src/util/testutil/integration/run.go:91
        	            				/src/util/testutil/integration/run.go:205
        	Error:      	Received unexpected error:
        	            	unsupported compression type nydus
        	            	github.com/moby/buildkit/util/stack.Enable
        	            		/src/util/stack/stack.go:77
        	            	github.com/moby/buildkit/util/grpcerrors.FromGRPC
        	            		/src/util/grpcerrors/grpcerrors.go:198
        	            	github.com/moby/buildkit/util/grpcerrors.UnaryClientInterceptor
        	            		/src/util/grpcerrors/intercept.go:41
        	            	google.golang.org/grpc.(*ClientConn).Invoke
        	            		/src/vendor/google.golang.org/grpc/call.go:35
        	            	github.com/moby/buildkit/api/services/control.(*controlClient).Solve
        	            		/src/api/services/control/control.pb.go:2208
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		/src/client/solve.go:258
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:75
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1598
        	            	failed to solve
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		/src/client/solve.go:273
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:75
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1598
        	Test:       	TestNydusIntegration/TestBuildExportNydusWithHybrid/worker=containerd

As we now build the integration tests image early we need to set BUILDKITD_TAGS at this step.