Resize Nightly icons to same scale as Stable/Preview (#4179)

Thorsten Ball created 

I'm sure @iamnbutler can do a better job here, this is just hacky, but
the size of the icon when doing `Cmd+tab` has been bugging me, so I
thought I'd resize them quickly to the dimensions the Stable/Preview
icons have.

Release Notes:

- N/A

Change summary

crates/zed/resources/app-icon-nightly.png    | 0 
crates/zed/resources/app-icon-nightly@2x.png | 0 
2 files changed

Detailed changes