Use the ARM runner for some 32-bit tests

This turns the `test-32bit` job into a matrix job definition with
two jobs: the i386 job, which already existed, and a new ARMv7 job,
which is able to run (still without emulation) in a container on
the new ARM64 runner.

As written, this is not expected to work, and is even expected to
break the old job, because I've commented out the code that
installs libraries for the 64-bit host architecture inside the
container to allow the 64-bit Node.js on the host to run in the
container for script steps. This is just to make sure that is still
needed, before attempting to generalize it to cover both platforms.

.github/workflows/ci.yml

@@ -199,9 +199,21 @@ jobs:
             etc/test-fixtures-windows-expected-failures-see-issue-1358.txt actual-failures.txt
 
   test-32bit:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        container-arch: [ i386, arm32v7 ]
+        include:
+          - container-arch: i386
+            runner-os: ubuntu-latest
+            toolchain: stable-i686-unknown-linux-gnu
+          - container-arch: arm32v7
+            runner-os: ubuntu-24.04-arm
+            toolchain: stable-armv7-unknown-linux-gnueabihf
+      fail-fast: false  # FIXME: Probably remove this.
+
+    runs-on: ${{ matrix.runner-os }}
 
-    container: i386/debian:stable-slim
+    container: ${{ matrix.container-arch }}/debian:stable-slim
 
     steps:
       - name: Prerequisites
@@ -214,17 +226,18 @@ jobs:
             git
             jq
             libssl-dev
-            libstdc++6:amd64  # To support external 64-bit Node.js for actions.
+            # libstdc++6:amd64  # To support external 64-bit Node.js for actions.
             pkgconf
           )
-          dpkg --add-architecture amd64
+          # dpkg --add-architecture amd64
           apt-get update
           apt-get install --no-install-recommends -y -- "${prerequisites[@]}"
         shell: bash
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: stable-i686-unknown-linux-gnu  # Otherwise it may misdetect based on the amd64 kernel.
+          # Avoid possible misdetection based on the 64-bit running kernel.
+          toolchain: ${{ matrix.toolchain }}
       - uses: Swatinem/rust-cache@v2
       - uses: taiki-e/install-action@v2
         with:
@@ -260,8 +273,9 @@ jobs:
         run: cross check -p gix --target ${{ matrix.target }}
       - name: Test (unit)
         run: |
-          # Run some high-level unit tests that exercise various pure Rust code to ease building test binaries.
-          # We would prefer `-p gix`. But with `cross`, fixture scripts try to run amd64 `git` as an armv7 binary.
+          # Run some high-level unit tests that exercise various pure Rust code to ease building
+          # test binaries. We would prefer `-p gix`. But with `cross`, fixture scripts try to run
+          # the host `git` as an armv7 binary.
           cross test -p gix-hashtable --target ${{ matrix.target }}
 
   lint: