ci: don't parallelize distcheck job

A while ago the `distcheck` CI job became flaky. This continued after
switching to Debian stable (from testing). Try stabilzing it by running
it single-threaded.

Closes #1339

.github/workflows/ci.yml

@@ -195,7 +195,7 @@ jobs:
       - name: 'autotools distcheck'
         if: ${{ matrix.target == 'distcheck' }}
         timeout-minutes: 10
-        run: make -C bld -j3 distcheck
+        run: make -C bld distcheck
       - name: 'cmake configure'
         if: ${{ matrix.build == 'cmake' }}
         run: |