$SHELL relative to $PATH prevents guake working

I'm using Guake 3.0.4 on Arch Linux, you can see all packages installed on my system and their versions [here](https://github.com/japgolly/.golly-rc/blob/f1fd796c6a3b94fd6b314cdeeebe611bf1992184/ref/desktop-arch/explicit_packages_and_versions).

I've removed my `~/.gconf/apps/guake` directory.

Trying to start `guake` fails with:
```
Logging configuration complete
DBus not running, starting it
Wanted GTK theme: ''
Prefer dark theme: False
reload_global: 'F12'
can't bind show-focus key
set_final_window_rect
height_percents = 50
width_percents = 100
halignment = 0
valignment = 0
vdisplacement = 0
hdisplacement = 0
Current monitor geometry
window_rect.x: 1200
window_rect.y: 0
window_rect.height: 1080
window_rect.width: 1920
is unity: False
Correcteed monitor size:
total_width: 1920
total_height: 1080
RESIZING MAIN WINDOW TO THE FOLLOWING VALUES:
window_rect.x: 1200
window_rect.y: 0
window_rect.height: 540
window_rect.width: 1920
Updated window position: (root_x=1200, root_y=0)
set_final_window_rect
height_percents = 50
width_percents = 100
halignment = 0
valignment = 0
vdisplacement = 0
hdisplacement = 0
Current monitor geometry
window_rect.x: 1200
window_rect.y: 0
window_rect.height: 1080
window_rect.width: 1920
is unity: False
Correcteed monitor size:
total_width: 1920
total_height: 1080
RESIZING MAIN WINDOW TO THE FOLLOWING VALUES:
window_rect.x: 1200
window_rect.y: 0
window_rect.height: 540
window_rect.width: 1920
Updated window position: (root_x=1200, root_y=0)
Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/dbus/bus.py", line 175, in activate_name_owner
return self.get_name_owner(bus_name)
File "/usr/lib/python3.6/site-packages/dbus/bus.py", line 361, in get_name_owner
's', (bus_name,), **keywords)
File "/usr/lib/python3.6/site-packages/dbus/connection.py", line 651, in call_blocking
message, timeout)
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.NameHasNoOwner: Could not get owner of name 'org.guake3.RemoteControl': no such name

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/guake/main.py", line 241, in main
remote_object = bus.get_object(DBUS_NAME, DBUS_PATH)
File "/usr/lib/python3.6/site-packages/dbus/bus.py", line 241, in get_object
follow_name_owner_changes=follow_name_owner_changes)
File "/usr/lib/python3.6/site-packages/dbus/proxies.py", line 248, in __init__
self._named_service = conn.activate_name_owner(bus_name)
File "/usr/lib/python3.6/site-packages/dbus/bus.py", line 180, in activate_name_owner
self.start_service_by_name(bus_name)
File "/usr/lib/python3.6/site-packages/dbus/bus.py", line 278, in start_service_by_name
'su', (bus_name, flags)))
File "/usr/lib/python3.6/site-packages/dbus/connection.py", line 651, in call_blocking
message, timeout)
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name org.guake3.RemoteControl was not provided by any .service files

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/bin/guake", line 10, in
sys.exit(exec_main())
File "/usr/lib/python3.6/site-packages/guake/main.py", line 356, in exec_main
if not main():
File "/usr/lib/python3.6/site-packages/guake/main.py", line 256, in main
instance = Guake()
File "/usr/lib/python3.6/site-packages/guake/guake_app.py", line 383, in __init__
self.add_tab()
File "/usr/lib/python3.6/site-packages/guake/guake_app.py", line 1751, in add_tab
pid = self.spawn_sync_pid(directory, box.terminal)
File "/usr/lib/python3.6/site-packages/guake/guake_app.py", line 1678, in spawn_sync_pid
Vte.PtyFlags.DEFAULT, wd, argv, [], GLib.SpawnFlags.DO_NOT_REAP_CHILD, None, None, None
GLib.Error: g-exec-error-quark: Failed to execute child process “zsh” (No such file or directory) (8)
```