pdm lock --check on a lockfile generated with older PDM version has a 0 exit code when there's a change in pyproject.toml

[chandrasekharan98]

• I have searched the issue tracker and believe that this is not a duplicate.

Steps to reproduce

1. Generate a lock file with an older version of PDM - it should generate a different lock file version (for eg: 4.4.1 got generated with PDM version 2.12.3)
2. Now make any change in pyproject.toml
3. Run this snippet to check if lockfile changes are necessary

if pdm lock --check; then
    echo "No changes required for pdm.lock in $pwd."
else
    echo "Lock file needs to be generated"
fi

Actual behavior

• Used PDM version 2.12.3 to generate a lock file versioned at 4.4.1
• Newer PDM versions such as 2.16.1 generate a lock file versioned at 4.4.2
• In case of running pdm lock --check after making pyproject.toml changes, it only throws the below warning but the exit code is 0
  WARNING: Lockfile is generated on an older version of PDM

Expected behavior

• This is not noticed if the lockfile versions are the same, i.e. if the lock file is generated with PDM version 2.16.1 for a lock_version of 4.4.2 and then we update the pyproject.toml - the exit code is 1 for the above snippet
  

Environment Information

(unstract-3.9) *[dont-merge/testing-pdm-lock-automation][~/zipstuff/unstract]$ pdm info && pdm info --env                                                                         [11:48:21] 
PDM version:
  3.16.1
Python Interpreter:
  /home/chandru/zipstuff/unstract/.venv/bin/python (3.9)
Project Root:
  /home/chandru/zipstuff/unstract
Local Packages:
  
INFO: PDM 2.16.1 is installed, while 2.19.1 is available.
Please run `pdm self update` to upgrade.
Run `pdm config check_update false` to disable the check.
{
  "implementation_name": "cpython",
  "implementation_version": "3.9.6",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "6.2.6-76060206-generic",
  "platform_system": "Linux",
  "platform_version": "#202303130630~1689015125~22.04~ab2190e SMP PREEMPT_DYNAMIC Mon J",
  "python_full_version": "3.9.6",
  "platform_python_implementation": "CPython",
  "python_version": "3.9",
  "sys_platform": "linux"
}
INFO: PDM 2.16.1 is installed, while 2.19.1 is available.
Please run `pdm self update` to upgrade.
Run `pdm config check_update false` to disable the check.

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