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

.gitlab-ci.yml

@@ -5,6 +5,12 @@
 default:
   image: "archlinux:latest"
 
+variables:
+  BUILD_SCRIPT: ./.gitlab/ci/build-inside-vm.sh
+  QEMU_BUILD_TIMEOUT: 2400
+  QEMU_VM_MEMORY: 3072
+  ARCHISO_COW_SPACE_SIZE: 2g
+
 stages:
   - lint
   - build
@@ -17,13 +23,15 @@ shellcheck:
     - make lint
 
 .build:
+  variables:
+    PACKAGE_LIST: dosfstools e2fsprogs erofs-utils jq libisoburn mtools openssl qemu-headless squashfs-tools zsync
   stage: build
   before_script:
-    - pacman -Syu --needed --noconfirm qemu-headless libisoburn
+    - pacman -Syu --needed --noconfirm qemu-headless
   script:
     - ./.gitlab/ci/build-host.sh
   after_script:
-    - cp -- "output/${PROFILE}/job-metrics" metrics.txt
+    - PROFILE=($BUILD_SCRIPT_ARGS) && cp -- "output/${PROFILE[0]}/metrics.txt" .
   artifacts:
     name: "output"
     paths:
@@ -38,12 +46,8 @@ build_short:
     matrix:
       # baseline does not support netboot with codesinging
       # https://gitlab.archlinux.org/archlinux/archiso/-/issues/132
-      - PROFILE: baseline
-        BUILDMODE:
-          - bootstrap
-      - PROFILE: releng
-        BUILDMODE:
-          - bootstrap
+      - BUILD_SCRIPT_ARGS: baseline bootstrap
+      - BUILD_SCRIPT_ARGS: releng bootstrap
 
 build_long:
   extends: .build
@@ -51,10 +55,6 @@ build_long:
     - fast-single-thread
   parallel:
     matrix:
-      - PROFILE: baseline
-        BUILDMODE:
-          - iso
-      - PROFILE: releng
-        BUILDMODE:
-          - iso
-          - netboot
+      - BUILD_SCRIPT_ARGS: baseline iso
+      - BUILD_SCRIPT_ARGS: releng iso
+      - BUILD_SCRIPT_ARGS: releng netboot