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.
Use the new name of the build's metrics file.

Fixes #134

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