envconfig: use $PKG_CONFIG for pkg-config find_program as well as pkgconfig
envconfig: use $PKG_CONFIG for pkg-config find_program as well as pkgconfig

It looks like internally we use pkgconfig, even though the installed
name is pkg-config. This fixes `tests cases/common/44 pkgconfig-gen`,
which will ignore PKG_CONFIG and select the wrong pkg-config binary if
you have $PKG_CONFIG set.

mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson