shell: Do not auto-detect manifest when '-p' is used.

Previous, "guix shell -p /path/to/profile" would have manifest/guix.scm
auto-detection turned on.

* guix/scripts/shell.scm (auto-detect-manifest)[options-contain-payload?]:
Return #t for 'profile.

guix/scripts/shell.scm

@@ -256,6 +256,7 @@ Return the modified OPTS."
       ((('package . _) . _) #t)
       ((('load . _) . _) #t)
       ((('manifest . _) . _) #t)
+      ((('profile . _) . _) #t)
       ((('expression . _) . _) #t)
       ((_ . rest) (options-contain-payload? rest))))