Extension development not working?

### Your environment

Which OS do you use:

NixOS 23.05, VSCodium installed with HomeManager

### Steps to reproduce

1. Clone this repo
2. Change something (I for example changed the `Finding haskell-language-server` message to `HEY Finding haskell-language-server`)
3. Press F5
4. The new editor shows up
5. It doesn't have the updated extension (In my case still just shows `Finding haskell-language-server` in the console, without the extra `HEY`)

I also tried just removing the extension from my global install, and if I then press F5 to test I still have no Haskell extension installed.

### Expected behaviour

Get the updated extension for testing

### Actual behaviour

I just have my normal setup, so I cannot test my changes to the extension :(