increase the number of mktemp template variables in download.sh

[tnthornton]

The call to mktemp in the download.sh script results in an error similar to the following when running the script from Alpine, in my case I was using busybox:

mktemp: Invalid argument

Which is easily reproducible outside of the download.sh script:

bash-4.3# mktemp -d /tmp/tmp.kubeflow-repo-XXXX
mktemp: Invalid argument

Once the number of template variables was increased to at least 6 places, the error was resolved:

bash-4.3# mktemp -d /tmp/tmp.kubeflow-repo-XXXXXX
/tmp/tmp.kubeflow-repo-XckJMKj

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This change is 

[k8s-ci-robot]

Hi @tnthornton. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[tnthornton]

/assign @jlewi

[jlewi]

Thank you so much for the fix
/ok-to-test

[jlewi]

/lgtm
/approve

[k8s-ci-robot]

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