Without this notify-osd fails to find dbus-binding-tool, since the
pkgconfig file would contain e.g.:
````
prefix=/nix/store/hxsbjbjn7g1j1cf60n228yi9wnzrl4yk-dbus-glib-0.104
exec_prefix=${prefix}
````
... and notify-osd is using `exec_prefix` to locate the binaries.
Set it to $dev to match the location of installed binaries (we have
`outputBin = "dev";`).
Issue #15074.
367b2aa1e4