Fix goal pose stamp

[redvinaa]

Follow-up on #4822

Description of contribution in a few bullet points

• There was a bug in the previous PR: the MPPI critics goal pose now uses stamp from global_path, which gets outdated by the TF buffer, and then fails to transform the goal pose.
• This doesn't come up if the plan is replanned frequently.
• As a solution, this PR uses the robot_pose.stamp, which gets updated frequently.

Description of documentation updates required from your changes

• None required

Description of how this change was tested

• In a custom simulation and on real robot, on the humble branch.

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Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[SteveMacenski]

I think this needs to be manually backported for Humble's version. I can do Jazzy/Rolling