Upstream switched use_system_libwayland to false [0] and
system_wayland_scanner_path will now only be declared if
use_system_wayland_scanner is set to true (it defaults to
use_system_libwayland) [1].
In Nixpkgs, we usually try to set use_system_* to true (i.e., we favor
system libraries over bundled/vendored ones) but in the case of Chromium
this can become difficult to maintain so we might eventually drop
`use_system_libwayland = true` again (IIRC this only caused one
incompatibility in the past though: b6b51374fc7; and f9d9864cb6 will
become relevant again when we build with the bundled libwayland).
[0]: b33bdfe265
[1]: 272220cefa
9d05d42f4d