services: Use more 'file-append'.

* gnu/services/authentication.scm (pam-ldap-pam-service): Use
'file-append' instead of #~(string-append ...).
* gnu/services/base.scm (greetd-pam-service): Likewise.
* gnu/services/kerberos.scm (pam-krb5-pam-service): Likewise.
* gnu/services/pam-mount.scm (pam-mount-pam-service): Likewise.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/authentication.scm

@@ -504,7 +504,7 @@ password.")
 (define (pam-ldap-pam-service config)
   "Return a PAM service for LDAP authentication."
   (define pam-ldap-module
-    #~(string-append #$(nslcd-configuration-nss-pam-ldapd config)
+    (file-append (nslcd-configuration-nss-pam-ldapd config)
                      "/lib/security/pam_ldap.so"))
   (pam-extension
     (transformer

gnu/services/base.scm

@@ -3263,7 +3263,7 @@ to handle."
   (define optional-pam-mount
     (pam-entry
      (control "optional")
-     (module #~(string-append #$greetd-pam-mount "/lib/security/pam_mount.so"))
+     (module (file-append greetd-pam-mount "/lib/security/pam_mount.so"))
      (arguments '("disable_interactive"))))
 
   (list

gnu/services/kerberos.scm

@@ -432,8 +432,8 @@ generates such a file.  It does not cause any daemon to be started.")))
    (transformer
     (lambda (pam)
       (define pam-krb5-module
-        #~(string-append #$(pam-krb5-configuration-pam-krb5 config)
+        (file-append (pam-krb5-configuration-pam-krb5 config)
-                         "/lib/security/pam_krb5.so"))
+                     "/lib/security/pam_krb5.so"))
 
       (let ((pam-krb5-sufficient
              (pam-entry

gnu/services/pam-mount.scm

@@ -94,7 +94,7 @@
   (define optional-pam-mount
     (pam-entry
      (control "optional")
-     (module #~(string-append #$pam-mount "/lib/security/pam_mount.so"))))
+     (module (file-append pam-mount "/lib/security/pam_mount.so"))))
   (list
    (pam-extension
     (transformer