ci: override ImageOS env var for aarch linux

The Elixir job has started failing because it says the `ImageOS` env var
is no longer one of a set of blessed values (it wants `ubuntu24` but
sees `ubuntu24-arm`). I'm guessing GitHub changed how they name the
runner images recently. This value is used to select the dependency
bundle to download from Hex, so overriding it like this is fine, at
least until the maintainers of the BEAM action update their
infrastructure to catch up with the GitHub rename, which will probably
cause this to break again.

Signed-off-by: Nathan Perry <nathan@tailscale.com>
Change-Id: I3cc65790ad65130486bb98c434b9c2f46a6a6964

Author: npry

.github/workflows/elixir.yml

@@ -84,13 +84,29 @@ jobs:
             ts_elixir/deps/
           key: ${{ matrix.platform.triple }}-mix-${{ env.cache_key }}-elixir-${{ matrix.elixir.version }}-otp-${{ matrix.otp.version }}-${{ hashFiles('**/mix.lock') }}
 
+      # The setup-beam action uses the `ImageOS` env var to select the dependency bundle to
+      # download from Hex, and so it needs to be one of a few blessed values. GitHub seems
+      # to have changed how they set this variable recently, including `-arm` for aarch64
+      # linux. This broke `setup-beam`, so we put it back how it was here and restore
+      # afterwards.
+      - name: Override ImageOS
+        if: ${{ matrix.platform.runner == 'linux-arm64-16cpu' }}
+        run: |-
+          echo ImageOSPrev="$ImageOS" >> $GITHUB_ENV
+          echo ImageOS=ubuntu24 >> $GITHUB_ENV
+
       - name: Install elixir
         id: install-elixir
         uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93 # 1.24.0
         with:
           otp-version: ${{ matrix.otp.version }}
           elixir-version: ${{ matrix.elixir.version }}
 
+      - name: Restore ImageOS
+        if: ${{ matrix.platform.runner == 'linux-arm64-16cpu' }}
+        run: |-
+          echo ImageOS="$ImageOSPrev" >> $GITHUB_ENV
+
       - name: Setup rust
         id: setup-rust
         uses: ./.github/actions/setup-rust