ci: override ImageOS=ubuntu24 on ARM64 release job

The Linux ARM64 release job (runs-on: ubuntu-24.04-arm) was
failing at the erlef/setup-beam step with:

  Tried to map a target OS from env. variable 'ImageOS' (got
  ubuntu24-arm64), but failed. [...supported: ubuntu22, ubuntu24,
  win19, win22, win25, macos13, macos14, macos15, macos26]

GitHub does support ARM runners, but the runner reports its
ImageOS as 'ubuntu24-arm64' which setup-beam's OS mapping
doesn't know about. The action's own error message tells us to
override 'ImageOS' to one of the supported values.

Override to 'ubuntu24' on the release job only. The Erlang/Elixir
prebuilt binaries from elixir-lang are architecture-independent
(Elixir ships separate aarch64 binaries for macOS but uses the
same x86_64-built ERTS via the linux x86_64 release on Linux
runners), so the ubuntu24 binaries work fine on the arm64
runner for our build purposes.

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

Author: gilbertwong96

.github/workflows/ci.yml

@@ -228,6 +228,13 @@ jobs:
         with:
           otp-version: ${{ env.OTP_VERSION }}
           elixir-version: ${{ env.ELIXIR_VERSION }}
+        env:
+          # On ubuntu-24.04-arm, ImageOS is reported as
+          # 'ubuntu24-arm64' which erlef/setup-beam doesn't recognize.
+          # Override to 'ubuntu24' (Erlang/Elixir prebuilt binaries
+          # are architecture-independent, so the ubuntu24 ones work
+          # fine on the arm64 runner).
+          ImageOS: ubuntu24
 
       - name: Install Zig
         # Burrito needs a specific zig version (0.16.0) to cross-compile