debuginfo: Set RUST_TEST_TASKS to 1 again for LLDB tests

Let's try if not running LLDB tests in parallel solves the sporadic deadlocks we've seen since enabling the LLDB test suite. Running the tests in parallel has lead to unstable behaviour in the past (with LLDB versions below 310.x.x). Maybe our new minimum LLDB version isn't quite up to it either.

cc @alexcrichton