[Github][libc] Also build arm container (#181497)

Some of the libc full build tests also run on AArch64 machines. We need
to build an ARM container or otherwise the container fails to start and
we never end up running anything.

Author: boomanaiden154

.github/workflows/build-libc-container.yml

@@ -19,7 +19,12 @@ jobs:
   build-libc-container:
     name: Build libc container
     if: github.repository_owner == 'llvm'
-    runs-on: ubuntu-24.04
+    runs-on: ${{ matrix.runs-on }}
+    strategy:
+      matrix:
+        runs-on:
+          - ubuntu-24.04
+          - ubuntu-24.04-arm
     steps:
       - name: Checkout LLVM
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1