Do not bust `poetry install` cache when manually installing pydantic v2. (#9407)

Using `poetry add` to install `pydantic@2.1` was also causing poetry to
change its lockfile. This prevented dependency caching from working:
- When attempting to restore a cache, it would hash the lockfile in git
and use it as part of the cache key. Say this is a cache miss.
- Then, it would attempt to save the cache -- but the lockfile will have
changed, so the cache key would be *different* than the key in the
lookup. So the cache save would succeed, but to a key that cannot be
looked up in the next run -- meaning we never get a cache hit.

In addition to busting the cache, the lockfile update itself is also
non-trivially long, over 30s:

![image](https://github.com/langchain-ai/langchain/assets/2348618/d84d3b56-484d-45eb-818d-54126a094a40)

This PR fixes the problems by using `pip` to perform the installation,
avoiding the lockfile change.