ci: fix BEAM setup on ubuntu-24.04-arm runner

GitHub's hosted ubuntu-24.04-arm runner reports ImageOS=ubuntu24-arm64,
which erlef/setup-beam does not recognize (it expects ubuntu24 and picks
the architecture from the runner separately). Split the setup-beam step so
the arm leg runs with env ImageOS=ubuntu24, keeping full Elixir/NIF test
coverage on arm64. Other platforms use their native ImageOS unchanged.

Author: bcardarella

.github/workflows/ci.yml

@@ -56,7 +56,20 @@ jobs:
       - name: Run Zig tests
         run: zig build test
 
+      # GitHub's hosted ubuntu-24.04-arm runner reports ImageOS=ubuntu24-arm64,
+      # which setup-beam doesn't recognize. Override it to ubuntu24 so the action
+      # can locate the arm64 OTP/Elixir builds (architecture is detected separately).
+      - name: Set up Erlang/Elixir (ARM)
+        if: matrix.os == 'ubuntu-24.04-arm'
+        uses: erlef/setup-beam@v1
+        env:
+          ImageOS: ubuntu24
+        with:
+          otp-version: "27"
+          elixir-version: "1.17"
+
       - name: Set up Erlang/Elixir
+        if: matrix.os != 'ubuntu-24.04-arm'
         uses: erlef/setup-beam@v1
         with:
           otp-version: "27"