substitute: Ignore exceptions thrown by 'set-thread-name'.

Fixes <https://bugs.gnu.org/32669>.
Reported by Ricardo Wurmus <rekado@elephly.net>.

* guix/scripts/substitute.scm (guix-substitute): Swallow 'system-error'
exceptions around 'set-thread-name' call.

guix/scripts/substitute.scm

@@ -1087,7 +1087,10 @@ default value."
     (#f     #f)
     (locale (false-if-exception (setlocale LC_ALL locale))))
 
-  (set-thread-name "guix substitute")
+  (catch 'system-error
+    (lambda ()
+      (set-thread-name "guix substitute"))
+    (const #t))                                   ;GNU/Hurd lacks 'prctl'
 
   (with-networking
    (with-error-handling                           ; for signature errors