dockerTools.tarsum: use current go

pkgs/build-support/docker/default.nix

@@ -132,6 +132,7 @@ rec {
     cp ${./tarsum.go} tarsum.go
     export GOPATH=$(pwd)
     export GOCACHE="$TMPDIR/go-cache"
+    export GO111MODULE=off
     mkdir -p src/github.com/docker/docker/pkg
     ln -sT ${docker.moby-src}/pkg/tarsum src/github.com/docker/docker/pkg/tarsum
     go build

pkgs/top-level/all-packages.nix

@@ -363,7 +363,6 @@ in
   grsync = callPackage ../applications/misc/grsync { };
 
   dockerTools = callPackage ../build-support/docker {
-    go = buildPackages.go_1_15;
     writePython3 = buildPackages.writers.writePython3;
   };