chore: Refine provider-integration GitHub workflows

[X-Guardian]

Description

Refine the provider-integration GitHub workflows:

1. Terraform cache path → ${RUNNER_TEMP}/terraform-cache

Was previously /usr/local/share/.cache/terraform, hard-coded as a Linux path. On Windows runners under Git Bash this resolves into a Git Bash–virtual path that doesn't actually exist on the Windows filesystem, producing this warning every run:

There are no cache folders on the disk

Replaced with ${RUNNER_TEMP}/... which is set by the runner on both Linux and Windows. Both linux_provider and windows_provider updated for consistency.

Side effect: Windows now actually gets a working terraform plugin cache (it never did before — the path was non-existent).

2. Fix windows_provider terraform cache key

Was using ${{ matrix.terraform }} while linux_provider used ${{ inputs.terraform_version }}. The matrix doesn't expose a terraform field, so the windows cache key was silently interpolating to an empty string — every windows matrix entry was sharing a single cache regardless of Terraform version. Aligned to use inputs.terraform_version like linux.

3. Remove unused steps

Remove 'Install pipenv' and 'Install Go' steps from the windows_provider workflow as they not used.

Checklist

• I have updated the PR title to match CDKTN's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes