backport of (#67046)

The installing-from-sources page for airflow-ctl had URLs pointing to a non-existent downloads.apache.org/airflowctl/ path. The correct path is downloads.apache.org/airflow/airflow-ctl/ (matching the base_url defined in conf.py) and downloads.apache.org/airflow/KEYS for the shared PGP key file.

Also fixes the bash script using three inconsistent variable names for the download directory — unified to ctl_download_dir which is the variable actually defined in the script.

Closes: #67025
(cherry picked from commit 09545ec)

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