go mod tidy for lotus-soup testplans

[nonsense]

Fixup for #6122

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Since CircleCI runs only go get lotus@master, but not go mod tidy, sometimes we have to clean the go.mod of old libraries in order to avoid compile errors:

Step 15/22 : RUN cd ${PLAN_DIR}     && go env -w GOPROXY="${GO_PROXY}"     && CGO_ENABLED=${CgoEnabled} GOOS=linux GOARCH=amd64 go build -o ${PLAN_DIR}/testplan.bin ${BUILD_TAGS} ${TESTPLAN_EXEC_PKG}
 ---> Running in d0cd81ce0cd5
�[91m/go/pkg/mod/github.com/filecoin-project/lotus@v1.8.1-0.20210428122447-4688da51781a/api/api_full.go:14:2: missing go.sum entry for module providing package github.com/filecoin-project/go-data-transfer (imported by github.com/filecoin-project/lotus/api); to add:
	go get github.com/filecoin-project/lotus/api@v1.8.1-0.20210428122447-4688da51781a
/go/pkg/mod/github.com/filecoin-project/lotus@v1.8.1-0.20210428122447-4688da51781a/node/modules/client.go:13:2: missing go.sum entry for module providing package github.com/filecoin-project/go-data-transfer/channelmonitor (imported by github.com/filecoin-project/lotus/node/modules); to add:
	go get github.com/filecoin-project/lotus/node/modules@v1.8.1-0.20210428122447-4688da51781a
/go/pkg/mod/github.com/filecoin-project/lotus@v1.8.1-0.20210428122447-4688da51781a/node/modules/client.go:14:2: missing go.sum entry for module providing package github.com/filecoin-project/go-data-transfer/impl (imported by github.com/filecoin-project/lotus/node/modules); to add:
	go get github.com/filecoin-project/lotus/node/modules@v1.8.1-0.20210428122447-4688da51781a
...