feat: build Arch packages for x86_64 and aarch64

Uses a matrix strategy with Go cross-compilation (GOARCH + CARCH)
so both architectures build in the same amd64 container.

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

.gitea/workflows/arch-publish.yml

@@ -11,6 +11,13 @@ jobs:
     container:
       image: archlinux:latest
       options: --platform linux/amd64
+    strategy:
+      matrix:
+        include:
+          - goarch: amd64
+            pkgarch: x86_64
+          - goarch: arm64
+            pkgarch: aarch64
     steps:
       - name: Install build dependencies
         run: pacman -Syu --noconfirm git nodejs go base-devel curl
@@ -33,6 +40,7 @@ jobs:
           pkgname="axolotl"
 
           sed -i "s/pkgver=.*/pkgver=$pkgver/" packaging/arch/PKGBUILD
+          sed -i "s/^arch=.*/arch=('${{ matrix.pkgarch }}')/" packaging/arch/PKGBUILD
 
           git archive --format=tar.gz --prefix="$pkgname-$pkgver/" -o "packaging/arch/$pkgname-$pkgver.tar.gz" HEAD
 
@@ -42,7 +50,7 @@ jobs:
           chown -R build:build .
 
       - name: Build package
-        run: su build -c "cd $PWD/packaging/arch && makepkg -s --noconfirm"
+        run: su build -c "cd $PWD/packaging/arch && GOARCH=${{ matrix.goarch }} CARCH=${{ matrix.pkgarch }} makepkg -s --noconfirm"
 
       - name: Publish to Gitea Registry
         run: |