Fix retention & location of temp dir via env vars

[danielrbradley]

Part 1: Allow using ENV vars for temp directory control

• Add specific environment variable to always retain directories and fix instruction that didn't work for passing tests to use the new variable.
• Document environment variables in readme and document decision process for retaining directories in code comments.
• Allow setting a default temp dir via env var - this would allow it to be configured more easily in CI so that we can grab failed test directories as workflow assets.

Example config file to use this locally when running in VSCode:

{
  "go.testTimeout": "320s",
  "go.testEnvVars": {
    "PULUMITEST_TEMP_DIR": "${workspaceFolder}/.tmp",
  }
}

Part 2: Run with custom temp dir in CI & capture failures

This highlighted that we need to make the initial directory creation safe (added a mutex & rewrote a bad test). This also sets up an example of how to execute tests in CI and capture the temp directory on failure.

GitHub actions example:

      - name: Run acceptance tests
        run: go test
        env:
          PULUMITEST_TEMP_DIR: ${{ github.workspace }}/test_temp
          PULUMITEST_RETAIN_FILES_ON_FAILURE: "true"

      - name: Upload test_temp directory on failure
        if: failure()
        uses: actions/upload-artifact@v4
        with:
          path: test_temp

Example test run with failing test: https://github.com/pulumi/providertest/actions/runs/13902023819?pr=127

[pulumi-bot]

This PR has been shipped in release v0.1.6.