Emscripten: More tightly scope fetching of sysroot

Prompted by #174

refresh.template.py

@@ -782,19 +782,6 @@ def _apple_platform_patch(compile_args: typing.List[str]):
     return compile_args
 
 
-def _get_sysroot(args: typing.List[str]):
-    """Get path to sysroot from command line arguments."""
-    for idx, arg in enumerate(args):
-        if arg == '--sysroot' or arg == '-isysroot':
-            if idx + 1 < len(args):
-                return pathlib.PurePath(args[idx + 1])
-        elif arg.startswith('--sysroot='):
-            return pathlib.PurePath(arg[len('--sysroot='):])
-        elif arg.startswith('-isysroot'):
-            return pathlib.PurePath(arg[len('-isysroot'):])
-    return None
-
-
 def _emscripten_platform_patch(compile_action):
     """De-Bazel the command into something clangd can parse.
 
@@ -805,8 +792,18 @@ def _emscripten_platform_patch(compile_action):
         return compile_action.arguments
 
     workspace_absolute = pathlib.PurePath(os.environ["BUILD_WORKSPACE_DIRECTORY"])
-    sysroot = _get_sysroot(compile_action.arguments)
-    assert sysroot, f'Emscripten sysroot not detected in CMD: {compile_action.arguments}'
+
+    def _get_sysroot(args: typing.List[str]):
+        """Get path to sysroot from command line arguments."""
+        for idx, arg in enumerate(args):
+            if arg == '--sysroot' or arg == '-isysroot':
+                if idx + 1 < len(args):
+                    return pathlib.PurePath(args[idx + 1])
+            elif arg.startswith('--sysroot='):
+                return pathlib.PurePath(arg[len('--sysroot='):])
+            elif arg.startswith('-isysroot'):
+                return pathlib.PurePath(arg[len('-isysroot'):])
+        return None
 
     def get_workspace_root(path_from_execroot: pathlib.PurePath):
         if path_from_execroot.parts[0] != 'external':
@@ -818,6 +815,8 @@ def get_workspace_root(path_from_execroot: pathlib.PurePath):
     environment['EMCC_SKIP_SANITY_CHECK'] = '1'
     environment['EM_COMPILER_WRAPPER'] = str(pathlib.PurePath({print_args_executable}))
     if 'EM_BIN_PATH' not in environment:
+        sysroot = _get_sysroot(compile_action.arguments)
+        assert sysroot, f'Emscripten sysroot not detected in CMD: {compile_action.arguments}'
         environment['EM_BIN_PATH'] = str(get_workspace_root(sysroot))
     if 'EM_CONFIG_PATH' not in environment:
         environment['EM_CONFIG_PATH'] = str(get_workspace_root(emcc_driver) / 'emscripten_toolchain' / 'emscripten_config')