golang: Prepare for future GOPATH removal

GOPATH is going to be removed in Go1.17 (see e.g. https://github.com/golang/go/issues/37755#issuecomment-771879911).

-> Prevent programs suddenly become installed into $HOME/go/bin instead of
gowork/bin, and mod cache to become something like $HOME/go/... instead of
being kept under gowork/

No change in behaviour for Go ≤ 1.16

component/golang/buildout.cfg

@@ -142,7 +142,7 @@ recipe	= slapos.recipe.template
 url     = ${:_profile_base_location_}/goenv.sh.in
 output	= ${gowork:directory}/env.sh
 depends = ${gowork.mkdir:recipe}
-md5sum	= 09c229292b341da0de933f5900de7011
+md5sum	= 6efdf5d63381c240c7d206d7939a63f7
 
 [gowork.mkdir]
 # NOTE do not use slapos.cookbook:mkdirectory here - if anything in software (not instance)

component/golang/goenv.sh.in

@@ -10,11 +10,14 @@ export PKG_CONFIG_PATH=$(echo -n "${gowork:cpkgpath}" |tr '\n' ':'):$PKG_CONFIG_
 # ---- 8< ----
 
 export GOPATH=$X:$GOPATH
-export PATH=$X/bin:$PATH
-export PS1="(`basename $X`) $PS1"
+export GOBIN=$X/bin
 export GOCACHE=$X/cache
+export GOMODCACHE=$X/pkg/mod
 export GOENV=$X/goenv
 
+export PATH=$GOBIN:$PATH
+export PS1="(`basename $X`) $PS1"
+
 # by default make go run in module-aware mode only if go.mod is present, and in GOPATH-mode otherwise.
 # (we still have some GOPATH uses; TODO -> default to "on" after GOPATH-mode usage is gone)
 export GO111MODULE=auto