fix: check if TARGET environment variable is set for e2e testing

[nestoracunablanco]

The end-to-end tests require the TARGET environment variable to be set. It is better to catch any failures related to this early on.

What this PR does / why we need it: Fail early

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Release note:

[openshift-ci]

Hi @nestoracunablanco. Thanks for your PR.

I'm waiting for a kubevirt 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.

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[0xFelix]

/ok-to-test

[0xFelix]

/approve

[kubevirt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 0xFelix

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[nestoracunablanco]

@0xFelix, @lyarwood, @geetikakay, is there anything else you need for the LGTM label? Thank you!

[0xFelix]

Ping @ksimon1 @akrejcir @jcanocan

[jcanocan]

Thanks for your contribution @nestoracunablanco!
/lgtm