buildGoPackage: re-enable GOCACHE

If projects uses go1.11 modules GOCACHE is required.
Also if buildGoPackage is used in a nix-shell setting we don't
want to override GOCACHE to allow incremental builds.
This should be backported to 18.09

pkgs/development/go-modules/generic/default.nix

@@ -103,6 +103,7 @@ go.stdenv.mkDerivation (
 
   '') + ''
     export GOPATH=$NIX_BUILD_TOP/go:$GOPATH
+    export GOCACHE=$TMPDIR/go-cache
 
     runHook postConfigure
   '';
@@ -193,9 +194,6 @@ go.stdenv.mkDerivation (
     find $bin/bin -type f -exec ${removeExpr removeReferences} '{}' + || true
   '';
 
-  # Disable go cache, which is not reused in nix anyway
-  GOCACHE = "off";
-
   shellHook = ''
     d=$(mktemp -d "--suffix=-$name")
   '' + toString (map (dep: ''