.gitlab/ci/build-host.sh:
Change the readonly TMPDIR variable to a global tmpdir variable and set it in the `init()` function.

.gitlab/ci/build-inside-vm.sh:
Change assigning the readonly tmpdir variable directly to assigning it after declaring it.
Change `cleanup()` and `create_zsync_delta()` to use bash-style statements and also check whether SUDO_GID is set before
using it.