ci: use ubuntu-24.04-arm runner for Linux ARM64 release

The Linux ARM64 release job was failing at the smoke test step
with 'Exec format error' because the binary was built for aarch64
but the smoke test ran on ubuntu-latest (x86_64).

GitHub provides ARM64 runners (Graviton-based) for public repos:
  - ubuntu-22.04-arm
  - ubuntu-24.04-arm

Switching the Linux ARM64 job to ubuntu-24.04-arm gives us a
native aarch64 environment where the binary can be smoke-tested
properly (./ado --version, etc.) instead of just checking 'file'
output.

All 4 release jobs now run on a native runner for their target
architecture:
  - Linux x86_64    -> ubuntu-latest      (x86_64)
  - Linux ARM64     -> ubuntu-24.04-arm   (aarch64)
  - macOS Intel     -> macos-latest       (x86_64)
  - macOS Apple     -> macos-latest       (aarch64)

Removed the matrix.native flag and the 'Verify cross-arch binary'
step — they're no longer needed. The smoke test step is
unconditional and includes a 'file $BINARY' line for an extra
sanity check on the produced binary.

Verified: actionlint clean, mix ci green, 162 tests pass.

Author: gilbertwong96

.github/workflows/ci.yml

@@ -201,7 +201,7 @@ jobs:
             ext: ""
             label: "Linux x86_64"
             artifact: ado-linux-x86_64
-          - os: ubuntu-latest
+          - os: ubuntu-24.04-arm
             target: linux_arm
             cpu: aarch64
             ext: ""
@@ -322,12 +322,16 @@ jobs:
           ls -lh release_bin/
 
       - name: Smoke test the binary
+        # All release jobs run on a native runner for the target
+        # platform (e.g. ubuntu-24.04-arm for the Linux ARM build),
+        # so the binary can always be executed here.
         run: |
           set -euo pipefail
           # Glob is intentional here — there's exactly one file in
           # the dir, but actionlint warns about unquoted globs.
           BINARY=$(ls release_bin/ado-*)
           echo "Testing $BINARY"
+          file "$BINARY"
           "$BINARY" --version || "$BINARY" --help | head -20
 
       - name: Upload release binary