You can install it from the Ubuntu 20.04 repositories.

wget http://mirrors.kernel.org/ubuntu/pool/universe/libz/libzip/libzip5_1.5.1-0ubuntu1_amd64.deb
sudo apt install ./libzip5_1.5.1-0ubuntu1_amd64.deb

https://github.com/actions/virtual-environments/issues/1084#issuecomment-648035263

This may help

(1) Locate a newer version of libzip and libzip-dev on Launchpad. Currently it's 1.5.1 under The Eoan Ermine. Download the correct deb files (libzip-dev_1.5.1-0ubuntu1_amd64.deb and libzip5_1.5.1-0ubuntu1_amd64.deb).

(2) Install them with

sudo dpkg -i libzip-dev_1.5.1-0ubuntu1_amd64.deb libzip5_1.5.1-0ubuntu1_amd64.deb

They may be missing some dependencies, you can try to install them with apt first, so you don't need to download any more "incompatible" packages from Launchpad. Then rerun this command.

(3) Install php-zip. Install PEAR (PECL) to upgrade php-zip to a newer version. Also install php-dev required by PECL to build php-zip.

sudo apt install php-zip php-pear php-dev

(4) Upgrade php-zip to support the newer libzip.

sudo pecl install zip

(5) Probably you don't need the development libraries any more.

sudo apt purge php-dev libzip-dev
sudo apt autoremove

(6) See phpinfo() if you have the correct libzip version. You may need to restart your webserver.

(7) You can prevent apt from automatically upgrading your php-zip, breaking your custom installation. But then you should remember to manually upgrade php with pecl, or check if the newer version supports the newer libzip. (See installed version with sudo dpkg -l | grep zip)

sudo apt-mark hold php<installed version>-zip

To get back the original version (assuming you've used libzip5, and you don't need the previously installed packages):

sudo apt-mark unhold php<installed version>-zip
sudo pecl uninstall zip
sudo apt purge php-pear libzip5 php-zip
sudo apt autoremove
sudo apt install php-zip

As suggested by the name mysql-workbench-community_8.0.16-1ubuntu18.04_amd64.deb was packaged for Ubuntu 18.04 which has libzip4. You can add the Ubuntu 18.04 package repository to your Ubuntu 19.04 installation like so:

$ echo deb http://archive.ubuntu.com/ubuntu/ bionic universe | sudo tee /etc/apt/sources.list.d/bionic.list
$ sudo apt-get update

And you will then be able to install with:

$ sudo apt install ./mysql-workbench-community_8.0.16-1ubuntu18.04_amd64.deb