Add projectDir argument to the configuration cache experiment github action

Currently the [experiment-config-cache](https://github.com/gradle/gradle-enterprise-build-validation-scripts/tree/main/.github/actions/gradle/experiment-config-cache/action.yml) expects to run the `./gradlew` command in the root of the workflow. This means that any repository that has `gradlew` executable in a subdirectory can't use this github action.

I propose we add an optional `projectDir` argument, that would allow users to select the directory to run the configuration cache experiment from, same as we have for other experiments done via scripts - [example: exp3](https://github.com/gradle/gradle-enterprise-build-validation-scripts/blob/55ab5fec9b9515712d497b862bd685a9b1000d74/.github/actions/gradle/experiment-3/action.yml#L17-L19).

Start with .github/actions/gradle/experiment-config-cache/action.yml and compare its inputs and working-directory handling with .github/actions/gradle/experiment-3/action.yml. Add the optional projectDir input so the configuration cache experiment can run from a selected directory, then verify the existing root-directory behavior remains unchanged.

Written by the indexing model from the issue text.