Publishing a function app changes LinuxFxVersion

Hi,
I'm publishing a linux function app that uses .NET 8 with the isolated runtime using the following command:
```
func azure functionapp publish my_function_app --build local
```
When I do this, I get the log message `Updating 'linuxFxVersion' to 'DOTNET-ISOLATED|6.0'.`, and sure enough if I check the function app on Azure, it's .NET version has been changed to 6, which then prevents the newly deployed app from starting up.

Is this a bug? If not, is there a way to not have the `func` command change settings?

Thanks!

Start by reproducing `func azure functionapp publish my_function_app --build local` with a Linux .NET 8 isolated function app and inspect when the command updates `linuxFxVersion`. Compare the deployed setting before and after publishing. Done means the publish flow preserves the app's configured runtime or provides a documented way to prevent this change.

Written by the indexing model from the issue text.