zed.desktop.in: Don't hardcode executable name due to binary conflicts (#12951)

> * There are a couple of other `zed` binaries that may be present on
linux systems
    ([1](https://openzfs.github.io/openzfs-docs/man/v2.2/8/zed.8.html),
[2](https://zed.brimdata.io/docs/commands/zed)). If you want to rename
our CLI
binary because of these issues, we suggest `zedit`, `zeditor`, or
`zed-cli`.

Due to aformentioned issue don't hardcode the executable name in the
.desktop file so envsubst can change it in accordance with the
distributor's requirement.

Resolves #12290.

Release Notes:

- N/A