component/wendelin.core: build with a zlib rpath

On wendlin.core 2, wcfs uses github.com/DataDog/czlib which uses zlib. We also
need to instruct linker to set a rpath for slapos' zlib, otherwise the system
one will be used.

Suggested by Kirill on nexedi/slapos!1007 (comment 136801)

component/wendelin.core/buildout.cfg

@@ -38,6 +38,12 @@ GO   = ${go:exe}
 # wcfs needs this:
 [gowork]
 cpkgpath += ${zlib:location}/lib/pkgconfig
+[wendelin.core-env]
+# `pkg-config --libs zlib` emits only -L, but not -Wl,-rpath
+# better set it via gowork:environment when "VAR += ..." support is there
+# environment +=
+#   CGO_LDFLAGS += -Wl,-rpath=${zlib:location}/lib
+CGO_LDFLAGS += -Wl,-rpath=${zlib:location}/lib
 
 
 [wendelin.core-repository]