chore: run buildpack integration test on pull request

[kenneth-rosario]

No description provided.

[KaylaNguyen]

Is this required for the test to use the right python version? Was the builder-runtime env not being used for this purpose?

[kenneth-rosario]

For some reason it was not picking up the correct version. See https://github.com/GoogleCloudPlatform/functions-framework-python/actions/runs/5073551086/jobs/9112706450#step:6:493 for example of the builder getting and using python v 3.11 when the action was asking for python37 . See https://github.com/GoogleCloudPlatform/functions-framework-python/actions/runs/5480718756/jobs/9984237010?pr=255#step:6:287 on example of how after adding file it is using the right version.

Not sure if we have been always testing with latest python version or if this bug appeared later on.

[kenneth-rosario]

I was able to reproduce by just running builder in local function

Steps to repro:

1. mkdir ~/my-repro cd ~/my-repro
2. main.py

import functions_framework

@functions_framework.http
def hello(request):
    return "Hello world!"

3. requirements file requirements.txt

functions-framework==3.4.0

4. Run buildpack
   pack build --builder gcr.io/buildpacks/builder:latest --env GOOGLE_FUNCTION_TARGET=hello--env X_GOOGLE_TARGET_PLATFORM=gcf --env GOOGLE_FUNCTION_SOURCE=. --env GOOGLE_RUNTIME=python37 --trust-builder --verbose my-first-function > test.log

5. It logs:
   Python version not specified, using the latest available version.

[kenneth-rosario]

@KaylaNguyen I modified conformance client to accept a runtime version flag that populates GOOGLE_RUNTIME_VERSION env var when building instead of doing a language specific way of pinning the version.