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.

.github/workflows/_test.yml

@@ -46,7 +46,11 @@ jobs:
               elif [ "${{ matrix.test_type }}" == "core-pydantic-2" ]; then
                 echo "Running core-pydantic-v2 tests, installing dependencies with poetry..."
                 poetry install
-                poetry add pydantic@2.1
+
+                # Install via `pip` instead of `poetry add` to avoid changing lockfile,
+                # which would prevent caching from working: the cache would get saved
+                # to a different key than where it gets loaded from.
+                poetry run pip install 'pydantic>=2.1,<3'
               else
                 echo "Running extended tests, installing dependencies with poetry..."
                 poetry install -E extended_testing
@@ -59,10 +63,10 @@ jobs:
             EXPECTED_VERSION=1
           fi
           echo "Checking pydantic version... Expecting ${EXPECTED_VERSION}"
-          
+
           # Determine the major part of pydantic version
           VERSION=$(poetry run python -c "import pydantic; print(pydantic.__version__)" | cut -d. -f1)
-          
+
           # Check that the major part of pydantic version is as expected, if not
           # raise an error
           if [[ "$VERSION" -ne $EXPECTED_VERSION ]]; then