Run link checks on 1 file at a time

markdown-link-check seems to have intermittent errors when checking
multiple files at a time. This change might be a workaround.

build/Makefile

@@ -153,7 +153,7 @@ $(LINKCHECK_OUTPUT): node_modules $(LINKCHECK_CONFIG)
  @mkdir -p $(GENERATED_DIR)
  @echo Checking Markdown links...
  cd ..; git ls-files -z "*.md" | \
- xargs -0 npx --prefix build/ markdown-link-check -c build/$(LINKCHECK_CONFIG) | \
+ xargs -0n1 npx --prefix build/ markdown-link-check -c build/$(LINKCHECK_CONFIG) | \
  tee build/$(LINKCHECK_OUTPUT); \
  exit $${PIPESTATUS[1]}