Detect conflict with extrepo

```console
# apt update
E: Conflicting values set for option Signed-By regarding source https://brave-browser-apt-release.s3.brave.com/ stable: /usr/share/keyrings/brave-browser-archive-keyring.gpg != /var/lib/extrepo/keys/brave_release.asc
E: The list of sources could not be read.
#
```

Start by reading install.sh and reproducing the reported apt update failure involving extrepo and the Brave source. Trace how the installer configures the Signed-By key and determine what behavior should avoid the conflicting values; done means apt can read the source list successfully.

Written by the indexing model from the issue text.