Respect Gemfile lockfile in bundle lock

[davidstosik]

Fixes #9565.

What was the end-user or developer problem that led to this PR?

As described in the issue linked above, when Gemfile specifies a custom lockfile name using lockfile, and the user runs bundle lock, the lock file gets written to Gemfile.lock instead of the specified name.

What is your fix for the problem, implemented in this PR?

Use definition.lockfile as a fallback instead of Bundler.default_lockfile.

You can confirm the problem easily:

$ mkdir /tmp/test_bundle && cd /tmp/test_bundle

$ bundle --version
4.0.15

$ cat > Gemfile <<-G 
gem "rails"
lockfile "custom.lock"
G

$ bundle lock
Writing lockfile to /private/tmp/test_bundle/Gemfile.lock
Resolving dependencies...

$ ls
Gemfile      Gemfile.lock

I also wrote a test that would fail without the fix in this PR, and now passes.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags