Let either test-32bit cancel the other on failure

Neither is expected to fail, so this is analogous to most of the
other matrix jobs, such as `test-fast`, with no more of a reason
than there to use `fail-fast: true`. So this removes that.

.github/workflows/ci.yml

@@ -211,7 +211,6 @@ jobs:
             runner-arch: arm64
             runner-os: ubuntu-24.04-arm
             toolchain: stable-armv7-unknown-linux-gnueabihf
-      fail-fast: false  # FIXME: Probably remove this.
 
     runs-on: ${{ matrix.runner-os }}