Fix Windows build failures caused by pathing issue in Gradle plugin tests

Created by: erichaagdev

This PR addresses Windows build failures caused by a pathing issue in Gradle plugin tests. In the Gradle plugin cacheability tests, rather than referencing the buildCacheDir directly, use an OS agnostic URI instead.

See this successful Windows job run: https://github.com/erichaagdev/openapi-generator/actions/runs/3273753945/jobs/5386375522

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
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