Use better defaults for installation directories (backport #574)

[mergify]

This is an automatic backport of pull request #574 done by Mergify.
Cherry-pick of cdf3202 has failed:

On branch mergify/bp/gvm-libs-21.04/pr-574
Your branch is ahead of 'origin/gvm-libs-21.04' by 1 commit.
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You are currently cherry-picking commit cdf3202.
  (fix conflicts and run "git cherry-pick --continue")
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Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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