ci: Consolidate build-host script

.gitlab/ci/build-host.sh:
Change all script-local variables to lower-case and make some of them overridable using environment variables (by
providing defaults).
Break down overly long commands by splitting them into a list of strings.
Use local variables where possible.
Change `main()` to use rsync instead of cp to copy the project to the build location more generically.
Change `main()` to use rsync instead of cp to copy the build artifacts on the VM from the project's build directory to
the output.
Remove all unnecessary `function` keywords for function declarations.
Replace the dependency on libisoburn's `xorriso` with libarchive's `bsdtar` and util-linux's `blkid` in
`prepare_boot()`.
Add `print_section_start()` and `print_section_end()` to reduce code duplication and error potential when printing lines
for gitlab's collapsible sections (https://docs.gitlab.com/ee/ci/jobs/#custom-collapsible-sections).
Document the script's behavior and expectations.
Document the understood environment variables and add links to documentation on understood units (in case of size
units).

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