Add Linux ARM64 (aarch64) AppImage build

Build the Linux AppImage for ARM64 (aarch64) alongside x86_64: the Linux CI
job now matrixes over both architectures, with arch-aware deps caching and
artifact/asset names (amd64 keeps its existing names). The aarch64 AppImage is
published to the nightly and release pages like the x86_64 one.

Run the unit-test suite on the aarch64 runner (faster GitHub arm runner); the
tests are built on that leg. Self-hosted keeps tests on the amd64 server.

.github/workflows/build_all.yml

@@ -55,11 +55,23 @@ jobs:
   build_linux:
     strategy:
       fail-fast: false
+      # Build both arches on every event (PRs included), through the same
+      # build_check_cache -> build_deps -> build_orca chain (the AppImage).
+      # aarch64 always uses the GitHub-hosted arm runner (there is no arm
+      # self-hosted server). amd64's empty arch is load-bearing: it keeps the
+      # historical 'linux-clang' deps cache key and the unsuffixed asset names.
+      matrix:
+        include:
+          - arch: ""
+            os: ${{ vars.SELF_HOSTED && 'orca-lnx-server' || 'ubuntu-24.04' }}
+          - arch: "aarch64"
+            os: ubuntu-24.04-arm
     # Don't run scheduled builds on forks:
     if: ${{ !cancelled() && (github.event_name != 'schedule' || github.repository == 'OrcaSlicer/OrcaSlicer') }}
     uses: ./.github/workflows/build_check_cache.yml
     with:
-      os: ${{ vars.SELF_HOSTED && 'orca-lnx-server' || 'ubuntu-24.04' }}
+      os: ${{ matrix.os }}
+      arch: ${{ matrix.arch }}
       build-deps-only: ${{ inputs.build-deps-only || false }}
     secrets: inherit
   build_windows:
@@ -99,7 +111,9 @@ jobs:
     secrets: inherit
   unit_tests:
     name: Unit Tests
-    runs-on: ${{ vars.SELF_HOSTED && 'orca-lnx-server' || 'ubuntu-24.04' }}
+    # Tests are built on the aarch64 leg by default (faster GitHub arm runner),
+    # so run them there; self-hosted builds them on the amd64 server instead.
+    runs-on: ${{ vars.SELF_HOSTED && 'orca-lnx-server' || 'ubuntu-24.04-arm' }}
     needs: build_linux
     if: ${{ !cancelled() && success() }}
     steps:
@@ -237,4 +251,3 @@ jobs:
         asset_name: OrcaSlicer-Linux-flatpak_nightly_${{ matrix.variant.arch }}.flatpak
         asset_content_type: application/octet-stream
         max_releases: 1 # optional, if there are more releases than this matching the asset_name, the oldest ones are going to be deleted
-