exernal_project: Fix default cross compile parameters
exernal_project: Fix default cross compile parameters

The variable `{host}` cannot be successfully expanded.
Using the `@HOST@` parameter, as suggested by the documentation, works.
mesonbuild/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,jpakkane/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,pexip/meson,pexip/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,pexip/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,pexip/meson,mesonbuild/meson