Switches testing with WebKitGTK to Ubuntu (removes the need for the extra dockerfile)
Previously (#18595 and #19466) a Dockerfile based on Debian was added for WebKitGTK testing with the idea of reusing the built products that build.webkit.org is producing.
That was done with the idea of having dozens of new nightly builds available at a day, which doesn't seem to match the needs for WPT testing/runs. It seems that having one new nightly a day (or even two a week) is more than enough. At that rate is more than reasonable to do new builds for testing with Ubuntu 18.04.
And it seems that harmonizing the testing of the Linux based browsers on the same docker image has several benefits, including making easier future maintenance.
Adds a nightly channel for testing with WebKitGTK on taskcluster.
The nighly channel downloads the last nightly tarball available at https://webkitgtk.org/built-products and installs it on taskcluster when the test start.
This tarball is generated using the webkitgtk internal JHBuild, that builds several third-party libraries needed for webkitgtk layout tests and then builds webkitgtk on top of this libraries.
Because of this, to use this tarball it is required to installing quite a lot of extra dependencies (that are needed by this extra third-party libraries).
A script is included inside the tarball to install this dependencies. This script is executed at test time in task cluster. But because installing them take quite a bit of time (several minutes), the dockerfile is modified to have them already installed and speed up this process. Having them already installed on the docker image is not strictly necessary, it just speed up the testing process.