Integration tests: use warehouse and role env vars

[sfc-gh-gbloom]

Pre-review checklist

• I've confirmed that instructions included in README.md are still correct after my changes in the codebase.
• I've added or updated automated unit tests to verify correctness of my new code.
• I've added or updated integration tests to verify correctness of my new code.
• I've confirmed that my changes are working by executing CLI's commands manually.
• I've confirmed that my changes are up-to-date with the target branch.
• I've described my changes in the release notes.
• I've described my changes in the section below.

Changes description

When we use the snowflake_session fixture in our integration tests, the config values are read from env variables prefixed with SNOWFLAKE_CONNECTIONS_INTEGRATION_.

This PR adds WAREHOUSE and ROLE to the list.
I also added .env to .gitignore as it's a common convention to put there local/test configuration that you don't want to check in.

Test

I noticed the problem when I tried to run the integration tests locally. I verified the values are correctly picked up after the change:

eval $(cat .env) python -m pytest -m integration

# .env content
SNOWFLAKE_CONNECTIONS_INTEGRATION_ACCOUNT="..."
SNOWFLAKE_CONNECTIONS_INTEGRATION_USER="..."
SNOWFLAKE_CONNECTIONS_INTEGRATION_PASSWORD="..."
SNOWFLAKE_CONNECTIONS_INTEGRATION_ROLE="..."
SNOWFLAKE_CONNECTIONS_INTEGRATION_WAREHOUSE="..."
...