[build]: Fix ARM build break introduced in PR# 7249 (sonic-net#7395)

PR# 7249 introduced a new bit of logic _after_ the point where the qemu based
build environment for ARM is removed. Hence the new logic fails when building
for ARM. Builds for AMD64 were not affected.

This commit moves the new logic introduced by PR# 7249 to just _before_ the
point where the qemu based build environment for ARM is removed. A comment is
added to reduce the likelihood of this sort of ARM build break from happening
again.

build_debian.sh

@@ -581,18 +581,21 @@ sudo mksquashfs $FILESYSTEM_ROOT $FILESYSTEM_SQUASHFS -e boot -e var/lib/docker
 
 scripts/collect_host_image_version_files.sh $TARGET_PATH $FILESYSTEM_ROOT
 
-if [ $MULTIARCH_QEMU_ENVIRON == y ]; then
-    # Remove qemu arm bin executable used for cross-building
-    sudo rm -f $FILESYSTEM_ROOT/usr/bin/qemu*static || true
-    DOCKERFS_PATH=../dockerfs/
-fi
-
 # Ensure admin gid is 1000
 gid_user=$(sudo LANG=C chroot $FILESYSTEM_ROOT id -g $USERNAME) || gid_user="none"
 if [ "${gid_user}" != "1000" ]; then
     die "expect gid 1000. current:${gid_user}"
 fi
 
+# ALERT: This bit of logic tears down the qemu based build environment used to
+# perform builds for the ARM architecture. This must be the last step in this
+# script before creating the Sonic installer payload zip file.
+if [ $MULTIARCH_QEMU_ENVIRON == y ]; then
+    # Remove qemu arm bin executable used for cross-building
+    sudo rm -f $FILESYSTEM_ROOT/usr/bin/qemu*static || true
+    DOCKERFS_PATH=../dockerfs/
+fi
+
 ## Compress docker files
 pushd $FILESYSTEM_ROOT && sudo tar czf $OLDPWD/$FILESYSTEM_DOCKERFS -C ${DOCKERFS_PATH}var/lib/docker .; popd