linux: don't use hardcoded path for Exec and Icon

Desktop files respect $PATH and $XDG_DATA_DIRS, so the full path does
not need to be specified to either.

This fixes the Flatpak build by allowing Flatpak to rewrite these to
match the app ID.

desktop/samplebrain.desktop

@@ -3,8 +3,8 @@ Encoding=UTF-8
 Type=Application
 Name=Samplebrain
 Comment=A sample masher designed by Aphex Twin
-Exec=/usr/bin/samplebrain
-Icon=/usr/share/icons/hicolor/scalable/apps/samplebrain.svg
+Exec=samplebrain
+Icon=samplebrain
 Terminal=false
 Categories=GNOME;Application;
 StartupNotify=true