Backport #24913

This pull request includes several changes to enhance the CI workflows and streamline the testing process for different platforms. The most important changes include the addition of new GitHub Actions for Linux and Nix testing, the consolidation of test result processing, and updates to the CI workflow configurations.

Enhancements to CI workflows:

• .github/workflows/linux-ci.yml: Added a new workflow configuration for Linux CI, including jobs for building PowerShell and running unelevated and elevated tests.
• .github/workflows/windows-ci.yml: Updated the workflow configuration to include all necessary paths and branches for triggering the CI.

Addition of new GitHub Actions:

• .github/actions/test/linux-packaging/action.yml: Created a new action for testing basic Linux packaging.
• .github/actions/test/nix/action.yml: Created a new action for testing PowerShell on non-Windows platforms.
• .github/actions/test/process-pester-results/action.yml: Created a new action to process Pester test results, convert them to CTRF format, and upload the artifacts.

Consolidation of test result processing:

• .github/actions/test/windows/action.yml: Replaced individual steps for converting, publishing, and uploading Pester test results with a single step that uses the new process-pester-results action.

Updates to test invocation scripts:

• tools/ci.psm1: Updated the Invoke-CITest and Invoke-LinuxTestsCore functions to include the OutputFormat parameter and skip rebuilding test tools when not necessary. [1] [2] [3] [4] [5] [6] [7]