[build_debian.sh] Freeze pip2 < version 21 (#6597)

**- Why I did it**

As per https://pypi.org/project/pip/ pip 21.0 does not not support Python 2 from Jan 2021. Most places in the codebase have already been pinned, but this one was missed.

**- How I did it**

Pin pip2 < version 21 in build_debian.sh

build_debian.sh

@@ -417,7 +417,7 @@ done < files/image_config/sysctl/sysctl-net.conf
 sudo augtool --autosave "$sysctl_net_cmd_string" -r $FILESYSTEM_ROOT
 
 # Upgrade pip via PyPI and uninstall the Debian version
-sudo https_proxy=$https_proxy LANG=C chroot $FILESYSTEM_ROOT pip2 install --upgrade pip
+sudo https_proxy=$https_proxy LANG=C chroot $FILESYSTEM_ROOT pip2 install --upgrade 'pip<21'
 sudo https_proxy=$https_proxy LANG=C chroot $FILESYSTEM_ROOT pip3 install --upgrade pip
 sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get purge -y python-pip python3-pip