When building with Meson, we cannot set environment variables while
running custom targets and our builddir layout is different from
Autotools anyway.

Now g-ir-scanner and friends can autodetect when they're being run
uninstalled by Meson and will find _giscanner.so and the giscanner
python files in the build directory. This is very similar to what
gdbus-codegen uses in glib/gio.

Same for girepository/gdump.c.