Attempt to install dependency on new Ubuntu CI images

[savetheclocktower]

Some CI jobs are complaining about X11…

make: Entering directory '/home/runner/work/pulsar/pulsar/node_modules/keyboard-layout/build'
  CXX(target) Release/obj.target/keyboard-layout-manager/src/keyboard-layout-manager.o
In file included from ../src/keyboard-layout-manager.cc:1:
../src/keyboard-layout-manager.h:7:10: fatal error: X11/Xlib.h: No such file or directory
    7 | #include <X11/Xlib.h>
      |          ^~~~~~~~~~~~
compilation terminated.

…so let's make sure it's installed.

Hoping it's just this easy!

[savetheclocktower]

I'm temporarily removing constraints on the “Validate WASM Grammar PR Changes” CI job just to confirm that it gets further into the process with this step added. Switching to draft until I can back out this change.

[savetheclocktower]

OK, got this job working again; re-imposed the constraint so that the “validate WASM grammar PR changes” CI job doesn't run for this change. You can look through the Actions tab's history to verify that there was a successful run of that job on this PR.

Any other CI failures are out of scope for this PR.

[DeeDeeG]

I'm a bit tempted to install the actual complete list of packages from the other CI job, it'd be a bit wasteful but reduce the chance, however small, of a difference affecting one job and not the other.

Also: Might be nice to clean up the commit history.

BOTH OPTIONAL. Approved as-is (and/or with the above suggestions done, assuming no new stuff beside that -- can re-review if there are changes beyond scope of this review feedback).

[savetheclocktower]

Also: Might be nice to clean up the commit history.

Squash and merge will take care of the commit history.

The ones I left out were the ones where I could be sure they were related to other tasks. xvfb and ffmpeg are related to capturing video of functional tests; squashfs is related to AppImages; et cetera. If this causes problems later I'll be happy to be assigned the PR to fix it. :)