Fix a deprecation warning printed by Circle-CI

Fix the following deprecation warning that appears in the Circle-CI
pipelines:

  go get: installing executables with 'go get' in module mode is deprecated.
          To adjust and download dependencies of the current module, use 'go get -d'.
          To install using requirements of the current module, use 'go install'.
          To install ignoring the current module, use 'go install' with a version,
          like 'go install example.com/cmd@latest'.
          For more information, see https://golang.org/doc/go-get-install-deprecation
          or run 'go help get' or 'go help install'.

Signed-off-by: Davide Madrisan <[email protected]>

Makefile

@@ -22,7 +22,7 @@ dev: prep
 bootstrap:
 	@for tool in  $(EXTERNAL_TOOLS) ; do \
 		echo "Installing/Updating $$tool" ; \
-		GO111MODULE=on go get -u $$tool; \
+		GO111MODULE=on go install $$tool; \
 	done
 
 cover: