Build the refreshed surface against the prefix the same run just installed

[estebanzimanyi]

functions/cgo.go says #cgo pkg-config: meos, so cgo resolves whichever
meos.pc the default search path answers with. The recipe set CGO_CFLAGS and no
PKG_CONFIG_PATH, so on any machine carrying an older /usr/local install that
stale meos.pc wins -- and it publishes an include path with none of the
-D=1 defines the fresh one carries.

Each family declaration then stays behind its #if and the build fails naming a
symbol the installed header does declare:

functions/meos_meos.go:956:2: could not determine kind of name
    for C.meos_initialize_pointcloud
functions/meos_meos.go:556:11: could not determine kind of name
    for C.rtree_create_tpcbox

which reads as a libmeos built without the family rather than as an environment
that never named the family at all. So the documented one-command refresh cannot
build the package it has just generated. CI escapes it only because it installs
libmeos INTO /usr/local, where the default pkg-config path finds the fresh one.

Naming PKG_CONFIG_PATH makes the build read the prefix this run installed.
MEASURED: pkg-config then reads
.meos-chain/prefix/lib/pkgconfig/meos.pc rather than
/usr/local/lib/pkgconfig/meos.pc, and publishes 11 family defines against 0;
all 13 meos headers the preamble includes resolve inside that prefix;
go build ./functions and go test ./tools/parity/ both exit 0 where the build
previously failed.

CGO_CFLAGS goes away because the same meos.pc supplies the include path and
cgo.go's own #cgo linux CFLAGS supplies the h3 one.