ci: use cross when enabled

Unbelievably, some of the steps in the main CI configuration were using
the hard-coded `cargo` instead of `${{ env.CARGO }}`. The latter will
interpolate to `cross` for targets requiring cross compilation. And
since all big endian targets are only tested via cross compilation, our
CI was not test big endian at all (beyond testing that compilation
succeeded).

This led to a [performance optimization] [breaking big endian] targets.

[performance optimization]: #151
[breaking big endian]: #152

.github/workflows/ci.yml

@@ -122,13 +122,13 @@ jobs:
     - name: Show byte order for debugging
       run: ${{ env.CARGO }} test --verbose $TARGET byte_order -- --nocapture
     - name: Run tests
-      run: cargo test --verbose
+      run: ${{ env.CARGO }} test --verbose
     - name: Run with only 'alloc' enabled
-      run: cargo test --verbose --no-default-features --features alloc
+      run: ${{ env.CARGO }} test --verbose --no-default-features --features alloc
     - name: Run tests without any features enabled (core-only)
-      run: cargo test --verbose --no-default-features
+      run: ${{ env.CARGO }} test --verbose --no-default-features
     - name: Run tests with miscellaneous features
-      run: cargo test --verbose --features logging
+      run: ${{ env.CARGO }} test --verbose --features logging
 
   # Setup and run tests on the wasm32-wasi target via wasmtime.
   wasm: