Fix env variable typo _AIRFLOW_WWW_USER_LASTNME

[brettpalmberg]

Env Variable Fix: _AIRFLOW_WWW_USER_LASTNME ---> _AIRFLOW_WWW_USER_LASTNAME

closes #18716

^ Add meaningful description above

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In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
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[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[kaxil]

Thanks @brettpalmberg