Use the latest bundle when testing solidus installation

[elia]

Summary

Whenever the gem or bundler versions change we should bust the cache key.
Also reach for the latest bundler version and get bug fixes and performance gains.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

Merging #5396 (ecdd51a) into main (e4ae444) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #5396   +/-   ##
=======================================
  Coverage   88.74%   88.74%           
=======================================
  Files         565      565           
  Lines       13986    13986           
=======================================
  Hits        12412    12412           
  Misses       1574     1574           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more