Update e2e-node-tests.md to include KUBE_ROOT

[ngopalak-redhat]

KUBE_ROOT environment variable is used while building node e2e tests. Failing to set this variable will result in the error:

I0327 08:36:58.617691    1793 build.go:59] Building k8s binaries...
make: Entering directory '/home/ubuntu/go/k8s.io/kubernetes'
make: *** No targets specified and no makefile found.  Stop.
make: Leaving directory '/home/ubuntu/go/k8s.io/kubernetes'
F0327 08:36:58.627303    1793 run_local.go:51] Failed to build the dependencies: unable to build cgo targets : failed to build go packages exit status 2
exit status 255

while running make test-e2e-node using the local or the remote method. The environment variable KUBE_ROOT is used here

Which issue(s) this PR fixes:

Fixes #

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ngopalak-redhat / name: Neeraj Krishna Gopalakrishna (e27f367)

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[palnabarun]

/ok-to-test

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[ylink-lfs]

/sig testing
/remove-lifecycle stale

[ylink-lfs]

Suggested change

	- *Clone the Kubernetes repository* into the default location: `~/k8s.io/kubernetes`.
	- *Clone the Kubernetes repository* into the default location: `$(go env GOPATH)/src/k8s.io/kubernetes`.