build: Disable parallel builds for now.

The sphinx-reredirect extension does not declare it self safe or unsafe for
parallel execution which leads to sphinx generating a warning when we build
for testing.

For now remove the parallel execution to get rid of the warning.

Longer Term: I've made a Merge request against the extension to fix the issue.

https://gitlab.com/documatt/sphinx-reredirects/-/merge_requests/10

and I've created #2107 to
track the issue so it doesn't get lost.

In the mean time, turning off the parallel builds doesn't seem to significantly
slow down the builds, a clean build from scratch without parallelization took less
than 6 minutes.

run_tests.sh

@@ -79,7 +79,7 @@ for project in "${projects[@]}"; do
     # -w writes warnings and errors to the specified file in
     #    addition to stderr.
     # -n runs in nit-picky mode.
-    make html SPHINXOPTS="-j4 -n -w $err_log_file"
+    make html SPHINXOPTS="-w $err_log_file"
 
     if [ $? -gt 0 ]; then
         project_build_status=1