Make get-kube.sh work properly the "ci/latest" pointer

[ixdy]

What this PR does / why we need it: this is a (late) followup from #36419, fixing a bug discovered in #36419 (comment).

Basically, get-kube-binaries.sh looks at $KUBERNETES_RELEASE_URL, but we weren't properly overriding it in get-kube.sh when downloading binaries from the CI release bucket. With this change, we set the variable correctly, and everything works:

$ KUBERNETES_RELEASE=ci/latest ~/code/kubernetes/src/k8s.io/kubernetes/cluster/get-kube.sh
Downloading kubernetes release v1.7.0-alpha.0.2068+3a3dc827e45426
  from https://dl.k8s.io/ci/v1.7.0-alpha.0.2068+3a3dc827e45426/kubernetes.tar.gz
  to /tmp/foo/kubernetes.tar.gz
Is this ok? [Y]/n

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   161  100   161    0     0   1004      0 --:--:-- --:--:-- --:--:--  1006
100 6023k  100 6023k    0     0  10.9M      0 --:--:-- --:--:-- --:--:-- 10.9M
Unpacking kubernetes release v1.7.0-alpha.0.2068+3a3dc827e45426
Kubernetes release: v1.7.0-alpha.0.2068+3a3dc827e45426
Server: linux/amd64  (to override, set KUBERNETES_SERVER_ARCH)
Client: linux/amd64  (autodetected)

Will download kubernetes-server-linux-amd64.tar.gz from https://dl.k8s.io/ci/v1.7.0-alpha.0.2068+3a3dc827e45426
Will download and extract kubernetes-client-linux-amd64.tar.gz from https://dl.k8s.io/ci/v1.7.0-alpha.0.2068+3a3dc827e45426
Is this ok? [Y]/n

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   161  100   161    0     0    991      0 --:--:-- --:--:-- --:--:--   987
100  348M  100  348M    0     0  39.1M      0  0:00:08  0:00:08 --:--:-- 34.2M
               
md5sum(kubernetes-server-linux-amd64.tar.gz)=e71c9b48f6551797a74de2b83b501c44
sha1sum(kubernetes-server-linux-amd64.tar.gz)=688dcf567b60e27e3d9bf97436154543432768cf
               
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   161  100   161    0     0   1019      0 --:--:-- --:--:-- --:--:--  1025
100 29.0M  100 29.0M    0     0  32.2M      0 --:--:-- --:--:-- --:--:-- 95.4M
               
md5sum(kubernetes-client-linux-amd64.tar.gz)=8e6a90298411ae5a0e943b1c0e182b1d
sha1sum(kubernetes-client-linux-amd64.tar.gz)=187a2d2c1c6ae1ead32ec4c1fa51f695223edaae
               
Extracting /tmp/foo/kubernetes/client/kubernetes-client-linux-amd64.tar.gz into /tmp/foo/kubernetes/platforms/linux/amd64
Add '/tmp/foo/kubernetes/client/bin' to your PATH to use newly-installed binaries.
Creating a kubernetes on gce...
...

Release note:

NONE

[k8s-reviewable]

This change is 

[ixdy]

cc @zreigz

[zreigz]

LGTM

[roberthbailey]

Is this var used elsewhere? Why is it exported now if it wasn't before?

[ixdy]

we need to set it in the environment for get-kube-binaries.sh to work properly for ci builds.

I reworked this a bit so we only inject it in the environment when calling get-kube-binaries.sh.

[roberthbailey]

/lgtm

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ixdy, roberthbailey

Needs approval from an approver in each of these OWNERS Files:

• cluster/OWNERS [roberthbailey]

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[ixdy]

@k8s-bot cvm gce e2e test this

[ixdy]

seriously?
@k8s-bot cvm gce e2e test this

[k8s-github-robot]

Automatic merge from submit-queue