Fix bug in gotestsum installer causing dependencies to not be downloaded

Building gotestsum started to fail after the repository removed some
dependencies on master.

What happens is that first, we `go get` the package (with go modules disabled);

    GO111MODULE=off go get -d gotest.tools/gotestsum

Which gets the latest version from master, and fetches the dependencies used
on master. Then we checkout the version we want to install (for example `v0.3.5`)
and run go build.

However, `v0.3.5` depends on logrus, and given that we ran `go get` for `master`,
that dependency was not fetched, and build fails.

This patch modifies the installer to use go modules (alternatively we could
probably run `go get .` after checking out the `v0.3.5` version),

We need to modify all installers, as it looks like this is a standard pattern
we use, but other dependencies were not failing (yet), so this patch only
addresses the immediate failure.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/dockerfile/install/gotestsum.installer

@@ -2,10 +2,10 @@
 
 : ${GOTESTSUM_COMMIT:=v0.3.5}
 
-install_gotestsum() {
-	echo "Installing gotestsum version $GOTESTSUM_COMMIT"
-	go get -d gotest.tools/gotestsum
-	cd "$GOPATH/src/gotest.tools/gotestsum"
-	git checkout -q "$GOTESTSUM_COMMIT"
+install_gotestsum() (
+	set -e
+	export GO111MODULE=on
+	go get -d "gotest.tools/gotestsum@${GOTESTSUM_COMMIT}"
 	go build -buildmode=pie -o "${PREFIX}/gotestsum" 'gotest.tools/gotestsum'
-}
+
+)