Revert "build-system/ruby: Use invoke."

This reverts commit 0244952c11.

We prefer 'invoke', but the custom error handling works better with the
code as-is.

guix/build/ruby-build-system.scm

@@ -143,13 +143,16 @@ GEM-FLAGS are passed to the 'gem' invokation, if present."
          (gem-dir (string-append vendor-dir "/gems/" gem-name)))
     (setenv "GEM_VENDOR" vendor-dir)
 
-    (or (apply invoke "gem" "install" gem-file
+    (or (zero?
+          ;; 'zero? system*' allows the custom error handling to function as
+          ;; expected, while 'invoke' raises its own exception.
+         (apply system* "gem" "install" gem-file
                 "--verbose"
                 "--local" "--ignore-dependencies" "--vendor"
                 ;; Executables should go into /bin, not
                 ;; /lib/ruby/gems.
                 "--bindir" (string-append out "/bin")
-               gem-flags)
+                gem-flags))
         (begin
           (let ((failed-output-dir (string-append (getcwd) "/out")))
             (mkdir failed-output-dir)