.gitlab/ci/build-inside-vm.sh:
Add `print_section_start()` and `print_section_end()` to remove code duplication when printing gitlab collapsible
sections (https://docs.gitlab.com/ee/ci/jobs/#custom-collapsible-sections).
Document further script dependencies.
Remove the temporary directory base (located in the project directory) instead of only the tempdir.
Simplify setting file and directory ownership when running with sudo, by only doing it once, recursively on the output
directory.
Make the script's output more verbose by using verbose flags for removal, ownership changes and zsyncmake actions and by
displaying e.g. created metrics and checksum files.
Change `create_metrics()` to output to a `metrics.txt` by default.
Change `create_checksums()` to create the checksums relative to the files.