build.run: use correct working directory in BuildPlan.execute_local.

This regression was introduced in commit 3200a3961. Fixes #1020.

amaranth/build/run.py

@@ -112,18 +112,16 @@ class BuildPlan:
                 # See https://stackoverflow.com/a/30736987 for a detailed explanation of why.
                 # Running the script manually from a command prompt is unaffected.
                 subprocess.check_call(["cmd", "/c", f"call {self.script}.bat"],
-                                        env=os.environ if env is None else env)
+                                      cwd=build_dir, env=os.environ if env is None else env)
             else:
                 subprocess.check_call(["sh", f"{self.script}.sh"],
-                                        env=os.environ if env is None else env)
+                                      cwd=build_dir, env=os.environ if env is None else env)
         # TODO(amaranth-0.5): remove
         if run_script is not None:
             warnings.warn("The `run_script` argument is deprecated. If you only want to "
                             "extract the files from the BuildPlan, use the .extract() method",
                             DeprecationWarning, stacklevel=2)
 
-
-
         return LocalBuildProducts(build_dir)