Fix CF CLI installation to use system version when available

[ivanovac]

What is this change about?

This PR fixes a critical issue in the CF CLI installation logic that causes integration test failures in BBL-based CI environments.

Problem

When running integration tests against a real CF environment (not Docker), the current util::tools::cf::install() function:

1. Always installs a fresh CF CLI to .bin/cf, even when CF CLI is already available in the system
2. Uses an outdated version (hardcoded CF v6.49.0 from 2020)
3. Overwrites the authenticated CF session created by the cf-space/login script
4. Results in error: No API endpoint set. Use 'cf login' or 'cf api' to target an endpoint.

This only affects the nodejs-buildpack because:

• The r-buildpack has a similar fix (added Nov 24, 2025)
• Our previous fix in PR Add check for existing CF CLI installation #856 was reverted by an automated github-config sync on Dec 9, 2025

Solution

This PR adds a check to detect if CF CLI is already installed before attempting to download it:

# Check if cf already exists in the target directory or system PATH
if [[ -f "${dir}/cf" ]] || command -v cf >/dev/null 2>&1; then
  util::print::title "CF CLI already installed (using system version)"
  cf version
  return 0
fi

Additionally:

• Removes the hardcoded version=6.49.0 parameter
• Fetches the latest stable CF CLI release instead
• Preserves the authenticated CF session from the CI environment

Testing

✅ Integration tests now pass in BBL environments (nodejs.buildpacks.ci.cloudfoundry.org)
✅ CF authentication is preserved throughout the test run
✅ System CF CLI v8.17.0 is detected and reused
✅ No "Installing cf" message appears when system CF is available

Related Issues

Fixes integration test failures in specs-edge-integration-master-cflinuxfs4 job
Related to PR #856 (reverted by github-config sync)
Matches the fix in r-buildpack (commit cce4022)

Follow-up Actions

⚠️ Important: This fix needs to be submitted to cloudfoundry/buildpacks-github-config repository to prevent future automated reverts. The daily github-config sync workflow will otherwise overwrite this change again.

Until the fix is merged in github-config, this PR should be maintained in the nodejs-buildpack to keep integration tests working.

Checklist
✅ Code follows the project's style guidelines
✅ Tests pass locally and in CI
✅ Documentation updated (inline comments added)
✅ Corresponding fix needs to be submitted to buildpacks-github-config