ci: Use environment variables to override build settings

.gitlab-ci.yml: Use environment variables to override the build settings offered by the build-host.sh script. Lower the RAM usage of the builds to 3072. Lower the COW space used by archiso to 2GiB. Remove dependency on libisoburn in the before_script section of the build target (libarchive and util-linux have replaced this requirement). Use size units in accordance with upstream documentation.

Fixes #134 (closed)

.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. 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).

.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.