Avoid changing working directory EMCC_BATCH_BUILD is unset. NFC

When not using `EMCC_BATCH_BUILD` the working directory does not
matter.

Author: sbc100

tools/system_libs.py

@@ -102,13 +102,13 @@ def clean_env():
   return safe_env
 
 
-def run_build_commands(commands, num_inputs, build_dir=None):
+def run_build_commands(commands, num_inputs, cwd=None):
   # Before running a set of build commands make sure the common sysroot
   # headers are installed.  This prevents each sub-process from attempting
   # to setup the sysroot itself.
   ensure_sysroot()
   start_time = time()
-  shared.run_multiple_processes(commands, env=clean_env(), cwd=build_dir)
+  shared.run_multiple_processes(commands, env=clean_env(), cwd=cwd)
   logger.info(f'compiled {num_inputs} inputs in {time() - start_time:.2f}s')
 
 
@@ -535,7 +535,9 @@ def build_objects(self, build_dir):
       objects.add(o)
       objects_lowercase.add(o.lower())
 
+    cwd = None
     if batch_inputs:
+      cwd = build_dir
       # Choose a chunk size that is large enough to avoid too many subprocesses
       # but not too large to avoid task starvation.
       # For now the heuristic is to split inputs by 2x number of cores.
@@ -547,7 +549,7 @@ def build_objects(self, build_dir):
           chunk_srcs = srcs[i:i + chunk_size]
           commands.append(building.get_command_with_possible_response_file(cmd + chunk_srcs))
 
-    run_build_commands(commands, num_inputs=len(objects), build_dir=build_dir)
+    run_build_commands(commands, num_inputs=len(objects), cwd=cwd)
     return objects
 
   def customize_build_cmd(self, cmd, _filename):