[bundler-checksum] Prevent Gem::LoadError by lazy loading json

What does this MR do and why?

🐛 Use lazy load to prevent Gem::LoadError

• JSON is lazy loaded because when require 'json' is top-of-file, it executes while the Gemfile is still being evaluated.
• That eagerly activates whatever json gem RubyGems finds first (e.g., 2.15.2), and when multiple json versions are installed, it may choose a version that conflicts with Gemfile.lock.
• When Bundler later tries to activate the locked json version (e.g., 2.13.1) check_for_activated_spec! raises Gem::LoadError, because a different version is already activated.
• Mitigated by postponing require 'json' until after Bundler finishes setup (i.e., right before parsing the checksum file).

References

Fixes this error encountered while attempting to re-checksum for a new gem addition:

Before

➜  bundle exec bundler-checksum init
bundler: failed to load command: bundler-checksum (/var/home/pboling/.local/share/mise/installs/ruby/3.3.9/lib/ruby/gems/3.3.0/bin/bundler-checksum)                                                                                                            
/var/home/pboling/.local/share/mise/installs/ruby/3.3.9/lib/ruby/gems/3.3.0/gems/bundler-2.7.1/lib/bundler/runtime.rb:317:in `check_for_activated_spec!': You have already activated json 2.15.2, but your Gemfile requires json 2.13.1. Prepending `bundle exec` to your command may solve this. (Gem::LoadError)
        from /var/home/pboling/.local/share/mise/installs/ruby/3.3.9/lib/ruby/gems/3.3.0/gems/bundler-2.7.1/lib/bundler/runtime.rb:25:in `block in setup'
# ...

After

➜  bundle exec bundler-checksum init
Initializing checksum file /var/home/pboling/src/forks/gitlab/Gemfile.checksum
Using CFPropertyList==3.0.7
Using RedCloth==4.3.4
Using acme-client==2.0.26
# ...

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