fix(rust): Run doctests serially, to prevent flaky builds from failed…

… doctest compiles

earthly/rust/scripts/std_build.py

@@ -330,9 +330,8 @@ def main():
         runner.run(cargo_lint, args.lint_flags, args.verbose)
 
         # Check if all Self contained tests pass (Test that need no external resources).
+        # But NOT doc tests, as these are not replacements for unit tests.
         if not args.disable_tests:
-            # Check if all documentation tests pass.
-            runner.run(cargo_doctest, args.doctest_flags, args.verbose)
             if args.cov_report == "":
                 # Without coverage report
                 runner.run(cargo_nextest, args.test_flags, args.verbose)
@@ -367,6 +366,15 @@ def main():
 
         results = runner.get_results()
 
+    # Check if all Self contained doc tests pass (Test that need no external resources).
+    # Can not be run in parallel with the normal builds as it becomes flaky and randomly fails.
+    # NOTE: DocTests are ONLY run to prove they are valid, they are NOT unit tests, and never
+    # currently contribute to code coverage.
+    if not args.disable_tests:
+        # Check if all documentation tests pass.
+        results.add(cargo_doctest(args.doctest_flags, args.verbose))
+
+
     results.print()
     if not results.ok():
         exit(1)