gst-docs doesn't track subproject changes across reconfiguration

Sequence of events:

meson build
ninja -C build
meson configure build -Dbad=disabled
ninja -C build

Results in

...
|Executing subproject gst-docs method meson 
|
|Project name: GStreamer manuals and tutorials
|Project version: 1.17.0.1
|C compiler for the host machine: ccache cc (gcc 9.2.0 "cc (GCC) 9.2.0")
|C linker for the host machine: GNU ld.bfd 2.33.1
|Message: Building gstreamer documentation
|Message: Building gst-plugins-base documentation
|Message: Building gst-plugins-good documentation

subprojects/gst-docs/meson.build:21:8: ERROR: Subproject "subprojects/gst-plugins-bad" required but not found.

This is ultimately due to the use of default_options to pass the list of projects into gst-docs to generate docs for which will not be updated on a configuration change.