ci: build Linux Arm AOT on Arm runners

Enabling Native AOT makes the Linux CI jobs link a native binary for
the target runtime. Cross-linking the Arm runtimes from the x64
ubuntu-latest runner has no target toolchain, so the linux-arm job
fails at the clang link step.

Run the Arm matrix entries on GitHub's Arm64 runners so linux-arm64
links natively, and install the armhf GNU toolchain
(gcc-arm-linux-gnueabihf, which pulls in binutils and the sysroot) for
the 32-bit linux-arm target, which still cross-compiles on the Arm64
host.

Assisted-by: Claude Opus 4.8
Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

.github/workflows/continuous-integration.yml

@@ -76,7 +76,7 @@ jobs:
 # ================================
   linux:
     name: Linux
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.runtime == 'linux-x64' && 'ubuntu-24.04' || 'ubuntu-24.04-arm' }}
     strategy:
       matrix:
         runtime: [ linux-x64, linux-arm64, linux-arm ]
@@ -89,6 +89,13 @@ jobs:
       with:
         dotnet-version: 10.0.x
 
+    - name: Install cross-compilation toolchain (arm)
+      if: matrix.runtime == 'linux-arm'
+      run: |
+        sudo apt-get update
+        # Brings in cross-compilation toolchain for armhf including binutils
+        sudo apt-get install -y gcc-arm-linux-gnueabihf
+
     - name: Install dependencies
       run: dotnet restore