services: bffe: Use guile from the package.

Rather than hardcoding a particular guile in the service definition.

* gnu/services/guix.scm (bffe-shepherd-services): Use guile from the package.
* gnu/packages/package-management.scm (bffe)[inputs]: Remove unnecessary
guile-next input.

gnu/packages/package-management.scm

@@ -992,8 +992,6 @@ transactions from C or Python.")
              guile-fibers-1.3
              guile-prometheus
              guile-lib))
-      (inputs
-       (list guile-next))
       (propagated-inputs
        (list guile-gnutls
              guile-json-4

gnu/services/guix.scm

@@ -1090,7 +1090,7 @@ ca-certificates.crt file in the system profile."
                   (append
                    (list #:pid-file "/var/run/bffe/pid")
                    #$(bffe-configuration-arguments config)))))
-     #:guile guile-3.0))
+     #:guile (lookup-package-native-input bffe-package "guile-next")))
 
   (match-record config <bffe-configuration>
     (package user group arguments extra-environment-variables)