fix(cli): honor --provider flag in create_crew() and fix loop variable shadow

[Beandon13]

Summary

• The for provider, env_keys in ENV_VARS.items() loop was silently overwriting the provider function parameter, making the --provider CLI flag a no-op
• The function always fell through to the interactive select_provider() prompt regardless of whether --provider openai was passed
• Renamed the loop variable to env_provider (eliminating the shadow) and added an early-out path that uses the supplied provider directly when it is non-None

Closes #5270

Testing

pytest lib/crewai/tests/cli/test_create_crew.py::test_provider_param_skips_interactive_selection -v

PASSED lib/crewai/tests/cli/test_create_crew.py::test_provider_param_skips_interactive_selection

[Beandon13]

Heads-up: this PR overlaps with open #5272 (and similarly-scoped closed #5401, #5274) on the loop-variable rename. The new behavior here on top of the rename is honoring the --provider flag end-to-end — without this PR, even after the rename, crewai create --provider openai still drops into the interactive select_provider() prompt. Happy to:

• close this in favor of fix: rename loop variable to avoid shadowing provider parameter in create_crew() #5272 if @joaomdmoura prefers the smaller patch + a follow-up for the flag wiring,
• or rebase on top of fix: rename loop variable to avoid shadowing provider parameter in create_crew() #5272 if it lands first,
• or split the flag-honoring change into its own PR if cleaner.

Either way the rename overlap is acknowledged.