gnu: lightdm: Explicitely use a native 'bash' for 'wrap-program'.

Having to use a native bash for 'wrap-program' is fairly
unusual, but here it is the right thing, as it is for
the test suite only.  Placate the 'wrapper-inputs' linter.

Also, eventually we might consider letting 'wrap-program'
bail out if no bash for the host is found, even when
compiling natively.  But that's for later.

* gnu/packages/display-managers.scm
  (lightdm)[arguments]<#:phases>{pre-check}: Set #:sh
  argument of 'pre-check'

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Change-Id: Icc60ef7ae24a9c5935d328ef72dd1779ef70b05f

gnu/packages/display-managers.scm

@@ -386,6 +386,7 @@ (define-public lightdm
           (add-before 'check 'pre-check
             (lambda _
               (wrap-program "tests/src/test-python-greeter"
+                #:sh (which "bash")
                 `("GUIX_PYTHONPATH"      ":" prefix (,(getenv "GUIX_PYTHONPATH")))
                 `("GI_TYPELIB_PATH" ":" prefix (,(getenv "GI_TYPELIB_PATH"))))
               ;; Avoid printing locale warnings, which trip up the text