Currently we can't build the in-progress GTK Linux embedding on bots, because we can't count on them having the necessary libraries. For LUCI, based on my discussion with the LUCI maintainers this is WAI; the expected solution is to build with a sysroot that has the necessary libraries (as Chromium does).

Because our buildroot was forked from Chromium, we have a bunch of sysroot plumbing. However, AFAIK it's never been used for host builds, so I don't know if it's complete/working.

We need to:

• Either create a sysroot image, or find a Chromium image that meets our needs (preferably the latter)
• Verify that the sysroot build plumbing works, and/or get it working. (E.g., we have the build/linux/sysroot_scripts/install-sysroot.py script mentioned in the page linked above, but it's 4 years old, so I'm sure we'll need to update some things)
• Plumb host sysroot support into the tools/gn wrapper in the engine repo (should be trivial once the above is done)
• Update bot recipes to pass the sysroot (or just make it the default in the step above, which would probably make life easier for engine developers anyway)

/cc @robert-ancell