[scripts] mount /proc in remaster_pre and umount in remaster_post

This avoids `find` to complain about Bad File Descriptor

scripts/remaster_post.sh

@@ -2,6 +2,13 @@
 PKGS_DIR="/sabayon/pkgcache"
 CHROOT_PKGS_DIR="${CHROOT_DIR}/var/lib/entropy/client/packages"
 
+# make sure to not leak /proc
+umount "${CHROOT_DIR}/proc" &> /dev/null
+umount "${CHROOT_DIR}/proc" &> /dev/null
+umount "${CHROOT_DIR}/proc" &> /dev/null
+
 echo "Merging back packages"
 cp "${CHROOT_PKGS_DIR}"/* "${PKGS_DIR}"/ -Ra
 rm -rf "${CHROOT_PKGS_DIR}"{,-nonfree,-restricted}/*
+
+exit 0