Remove LD_LIBRARY_PATH before running gdb (#13594)

Suggestion by @peace-maker

tests/stubtest_third_party.py

@@ -252,6 +252,9 @@ def setup_gdb_stubtest_command(venv_dir: Path, stubtest_cmd: list[str]) -> bool:
         import sys
 
         stubtest_env = os.environ | {{"STUBTEST_ARGS": json.dumps(sys.argv)}}
+        # With LD_LIBRARY_PATH set, some GitHub action runners look in the wrong
+        # location for gdb, causing stubtest to fail.
+        stubtest_env.pop("LD_LIBRARY_PATH", None)
         gdb_cmd = [
             "gdb",
             "--quiet",