curl-based installer is not idempotent

- EdgeDB Version:
- EdgeDB CLI Version: 3.3.0+071876c
- OS Version: Ubuntu 23.04 "Lunar Lobster"

Steps to Reproduce:

1. View the instructions found at https://www.edgedb.com/install
2. curl --proto '=https' --tlsv1.2 -sSf https://sh.edgedb.com > install-edgedb.sh
3. run install-edgedb.sh to install the CLI
4. run install-edgedb again
"edgedb: error".

Other options that would be better:

1. The installer could be idempotent
2. If you try to run it on top of an install it could refuse and tell the user that the program already exists
3. Ask if the user is trying to reinstall and overwrite the existing install?
4. Require the user to uninstall before running the script.

Start by reproducing the failure with the installer downloaded from https://sh.edgedb.com and the commands in the issue, then inspect the installer entry point in the gel-cli repository. Confirm with maintainers which existing-install behavior is wanted from the listed options, and verify that running the installer again follows that agreed behavior without the current error.

Written by the indexing model from the issue text.