Adjust cross CI jobs for changed just recipe syntax

Also, since `--test-threads 1` didn't reveal anything different
locally, this uses the nicknamed convenience recipes.

Author: EliahKagan

.github/workflows/ci.yml

@@ -334,7 +334,12 @@ jobs:
 
     strategy:
       matrix:
-        target: [ armv7-linux-androideabi, s390x-unknown-linux-gnu ]
+        nickname: [ android, s390x ]
+        include:
+          - nickname: android
+            target: armv7-linux-androideabi
+          - nickname: s390x
+            target: s390x-unknown-linux-gnu
       fail-fast: false
 
     steps:
@@ -356,7 +361,7 @@ jobs:
       - name: Test (unit)
         env:
           RUST_BACKTRACE: '1'
-        run: just cross-test ${{ matrix.target }}
+        run: just cross-test-${{ matrix.nickname }}
 
   lint:
     runs-on: ubuntu-latest