test: Return root module root dir when using go.work

[dlipovetsky]

What problem does this PR solve?:
Fixes the test helper when go.work is used locally.

Previously, the helper returned all the output of go list -m, and when using go.work, the output contains multiple lines, with the directory of the root module on the first line. Now, the helper returns only the first line.

The previous behavior caused the test to fail:

=== Failed
=== FAIL: pkg/handlers/generic/lifecycle/ccm/nutanix Test_templateValues (0.00s)
    handler_test.go:71:
        	Error Trace:	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/pkg/handlers/generic/lifecycle/ccm/nutanix/handler_test.go:193
        	           				/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/pkg/handlers/generic/lifecycle/ccm/nutanix/handler_test.go:71
        	Error:      	Received unexpected error:
        	           	open /home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix
        	           	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/api
        	           	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/common
        	           	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/docs
        	           	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/hack/third-party/caaph
        	           	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/hack/third-party/capa
        	           	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/hack/third-party/capx
        	           	/home/dlipovetsky/projects/cluster-api-runtime-extensions-nutanix/hack/tools/charts/cluster-api-runtime-extensions-nutanix/addons/ccm/nutanix/values-template.yaml: no such file or directory
        	Test:       	Test_templateValues

DONE 446 tests, 1 failure in 35.288s
make: *** [make/go.mk:49: test.root] Error 1

Which issue(s) this PR fixes:
Fixes #

How Has This Been Tested?:

The test passes. My go.work:

go 1.24.2

use (
	.
	./api
	./common
	./docs
	./hack/third-party/caaph
	./hack/third-party/capa
	./hack/third-party/capx
	./hack/tools
)

Special notes for your reviewer:

[jimmidyson]

Thanks!