Set the target explicitly when running tests

Without this, we always tested against the default target triple of
the host, even when the job was supposed to test a cross-compiled
build.

.github/workflows/test.yml

@@ -17,7 +17,7 @@ jobs:
           # FIXME: these jobs are falling back to x86-64, so they're not testing anything.
           #
           # See also https://github.com/cross-rs/cross-toolchains#apple-targets
-          # - { target: aarch64-unknown-linux-gnu, os: ubuntu-20.04, use-cross: true }
+          - { target: aarch64-unknown-linux-gnu, os: ubuntu-20.04, use-cross: true }
           # - { target: aarch64-apple-darwin, os: macos-latest, use-cross: true }
     env:
       BUILD_CMD: cargo
@@ -44,7 +44,7 @@ jobs:
 
     - name: Test
       shell: bash
-      run: $BUILD_CMD test
+      run: $BUILD_CMD test --target ${{ matrix.job.target }}
 
   test_mime_db:
     name: Test with MIME database