Rename the observations variable in the evaluation util to avoid shadowing

[npit]

This is a simple renaming. It enables a callback in evaluate_policy to have access to the observation vector that is fed to the environment step function, which is currently shadowed by the output observation.

Description

Motivation and Context

Closes #1287

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[araffin]

LGTM =)
Could you update the changelog and do a PR to update SB3 contrib repo too? (see PR template)

Sorry for the delay, I was focused on the gym/gymnasium transition...

[npit]

Could you update the changelog and do a PR to update SB3 contrib repo too? (see PR template)

Sorry about that, I missed both of these...
Updated the changelog and created a similar PR to the contrib repo.

[araffin]

LGTM, please apply the changes I did to the SB3 branch