linux: don't presume install location in desktop file

The app.desktop file shipped with the default build presumes the install
location of the binary. Let the OS determine the executable through it's
normal PATH variables.

dist/linux/app.desktop

@@ -2,7 +2,7 @@
 Name=Ghostty
 Type=Application
 Comment=A terminal emulator
-Exec=/usr/bin/ghostty
+Exec=ghostty
 Icon=com.mitchellh.ghostty
 Keywords=terminal;tty;pty;
 StartupNotify=true
@@ -11,4 +11,4 @@ Actions=new-window
 
 [Desktop Action new-window]
 Actions=new-window
-Exec=/usr/bin/ghostty
+Exec=ghostty