revert to installing gtest sources for alpine and rpm

Closes #3350 (closed) a second time.

This is needed to make --with-gtest-sources pass in jenkins. There was a false misconception that all gtest packages install sources in /usr/src/googletest. The reality is that alpine and rpm packages install shared libraries instead.

• ut-basic failure on kea-dev: https://jenkins.aws.isc.org/job/kea-dev/job/ut-basic/1482/
• ut-extended failure on kea-dev: https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/1450/
• tried ut-basic, but it fails because there is no prepare-system, we'll recreate the AMIs: https://jenkins.aws.isc.org/job/kea-manual/job/ut-basic/31/
• ut-extended should pass: https://jenkins.aws.isc.org/job/kea-manual/job/ut-extended/304/