Change Poetry install directory

[szb640]

What does this Pull Request accomplish?

The nibcq-python project requires a dedicated GitHub agent to run unit tests. When running the setup-poetry action a second time on the agent, the following error is thrown:

ln: failed to create symbolic link 'C:\WINDOWS\ServiceProfiles\NetworkService\.local\poetry\bin/poetry.exe': File exists

This PR addresses the issue, by changing the target directory of the Poetry installation, from a home directory that's shared between runs to a .cache subfolder in the project repository, that is cleaned by GitHub during checkout.

Why should this Pull Request be merged?

This change allows the setup-poetry action to be used on self-hosted runners.

What testing has been done?

Running the pipeline with the changes included produced a passing build:

• Failing build without fix.
• Passing build with fix.
• Verified caching occurs when building for second time.

The pipeline included the following use cases:

• Ephemeral runners on Linux
• Self-hosted runners on Windows

[mshafer-NI]

This change makes sense to me. We did find during our migration to poetry2 that there's another variable where poetry will store things on install (such as any plugins), so we should probably also point that to the runner temp dir.