ci: cache packages

Results in ~40s saved in each job.

Signed-off-by: Filipe Laíns <lains@archlinux.org>

.gitlab-ci.yml

@@ -1,12 +1,19 @@
 variables:
   MAKEFLAGS: "-j10"
   VERBOSE: 1
+  PACMAN_OPTS: --needed --noconfirm --cachedir .pkg-cache
+
+cache:
+  key: pkgs-v1
+  paths:
+    # For some reason Gitlab CI only supports storing cache/artifacts in a path relative to the build directory
+    - .pkg-cache
 
 .arch-test:
   image: archlinux/base
   before_script:
     - >
-      pacman -Syu --needed --noconfirm
+      pacman -Syu $PACMAN_OPTS
       base-devel git
       gpgme libarchive curl
       python
@@ -30,14 +37,14 @@ arch-debug:
 arch-docs:
   extends: .arch-test
   script:
-    - pacman -Syu --needed --noconfirm asciidoc
+    - pacman -Syu $PACMAN_OPTS asciidoc
     - meson -Ddoc=enabled build
     - ninja -C build
 
 arch-clang:
   extends: .arch-test
   script:
-    - pacman -Syu --needed --noconfirm clang
+    - pacman -Syu $PACMAN_OPTS clang
     - CC=clang meson build
     - ninja -C build
     - fakechroot meson test -C build
@@ -80,6 +87,7 @@ arch-no-nls:
 
 debian:
   image: debian:bullseye
+  cache: {}
   before_script:
     - apt update
     - >
@@ -95,6 +103,7 @@ debian:
 
 fedora:
   image: fedora
+  cache: {}
   before_script:
     - >
       dnf -y install