ci: pacman-key --init/--populate before keyring upgrade

archlinux:latest slim image has no local pacman master key and an
unpopulated upstream keyring, so:
  - the archlinux-keyring upgrade fails with "no secret key available to
    sign with" because pacman can't sign the keyring it's rewriting
  - falling through to -Syu hits the original "unknown trust" errors on
    libseccomp and zip

Run pacman-key --init then --populate archlinux before any pacman -S.
This is the documented bootstrap for slim Arch CI containers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/ci.yaml

@@ -13,10 +13,17 @@ jobs:
     steps:
       - name: Install build dependencies
         run: |
-          # archlinux:latest images ship a snapshot keyring; new packages
-          # signed after that snapshot fail PGP verification. Refresh the
-          # keyring first, then everything else.
-          pacman -Sy --noconfirm archlinux-keyring
+          # The slim archlinux:latest image arrives without a populated
+          # pacman keyring AND without a local pacman master key, so:
+          #   1. --init  generates the local signing key (needed when pacman
+          #      itself rewrites the keyring during package upgrades)
+          #   2. --populate archlinux  imports the upstream master keys so
+          #      currently-shipping signatures verify
+          # After that we can refresh archlinux-keyring to pick up keys for
+          # packagers added after the image's snapshot, then -Syu the rest.
+          pacman-key --init
+          pacman-key --populate archlinux
+          pacman -Sy --noconfirm --needed archlinux-keyring
           pacman -Syu --noconfirm --needed \
             base-devel git zip tar \
             clang lld libc++ cmake \