fix(steps-guix): run finalize_fhs after linux64 jump and drop ad-hoc mtab hack

steps-guix/improve/after.sh

@@ -2,9 +2,4 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-mkdir -p /etc
-if [ ! -e /etc/mtab ]; then
-    ln -s /proc/mounts /etc/mtab
-fi
-
 exec /steps/improve/after.sh

steps-guix/improve/finalize_fhs.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+exec /steps/improve/finalize_fhs.sh

steps-guix/manifest

@@ -11,6 +11,7 @@ build: musl-obstack-1.2.3
 build: elfutils-0.194
 build: linux-6.12.74
 jump: linux64
+improve: finalize_fhs
 improve: get_network
 build: libgpg-error-1.59
 build: libgcrypt-1.12.1