Don't run install in parallel on macOS CI

There seems to be a race condition in macOS's install command, such that
if two install -d commands simultaneously try to create the same
directory, one of them returns an error. This could happen when running
the install and install-desktop-file makefile targets at the same time.

.github/workflows/ci.yml

@@ -467,7 +467,7 @@ jobs:
           make check CHECK_OPTIONS=-v | tee /dev/stderr | grep -q '${{ matrix.build.failing_tests }} test(s) have FAILED'
         fi
     - name: install test
-      run: make prefix= DESTDIR="${PWD}" ${{ matrix.build.install_target }}
+      run: make prefix= DESTDIR="${PWD}" -j 1 ${{ matrix.build.install_target }}
 
   build-osx12:
     runs-on: macos-12
@@ -532,7 +532,7 @@ jobs:
           make check CHECK_OPTIONS=-v | tee /dev/stderr | grep -q '${{ matrix.build.failing_tests }} test(s) have FAILED'
         fi
     - name: install test
-      run: make prefix= DESTDIR="${PWD}" ${{ matrix.build.install_target }}
+      run: make prefix= DESTDIR="${PWD}" -j 1 ${{ matrix.build.install_target }}
 
   build-osx11:
     runs-on: macos-11
@@ -597,7 +597,7 @@ jobs:
           make check CHECK_OPTIONS=-v | tee /dev/stderr | grep -q '${{ matrix.build.failing_tests }} test(s) have FAILED'
         fi
     - name: install test
-      run: make prefix= DESTDIR="${PWD}" ${{ matrix.build.install_target }}
+      run: make prefix= DESTDIR="${PWD}" -j 1 ${{ matrix.build.install_target }}
 
   build-fedora39:
     runs-on: ubuntu-latest