Reduce parallelism in CI builds

I'm hoping this will prevent the
```
g++: fatal error: Killed signal terminated program cc1plus
```

errors I've been seeing.

.ci/cxx-tests.sh

@@ -1,7 +1,8 @@
 #!/bin/sh
 set -e
 
-if ! make -C build -j check; then
+# -j4 rather than -j to avoid getting killed on Github Actions
+if ! make -C build -j4 check; then
   cat build/src/test-suite.log
   exit 1
 fi

.github/workflows/test.yml

@@ -41,7 +41,7 @@ jobs:
       - name: Run configure
         run: ./.ci/configure.sh ${{ matrix.extras }}
       - name: Build
-        run: make -C build -j
+        run: make -C build -j4
       - name: Run tests
         run: ./.ci/cxx-tests.sh
       - name: Show ccache stats
@@ -152,7 +152,7 @@ jobs:
       - name: Run configure
         run: ./.ci/configure.sh yes --disable-optimized --enable-coverage
       - name: Build
-        run: make -C build -j
+        run: make -C build -j4
       - name: Run C++ tests
         run: ./.ci/cxx-tests.sh
       - name: Collect coverage