test: add -go flag to go tool fix

Add -go flag to go tool fix to correctly set the go version.
A change external to the codebase is causing the default go version
to not populate.

fixes: coreos#1950

test

@@ -44,7 +44,9 @@ PKG_VET=$(go list ./... | \
 	grep --invert-match internal/log)
 
 echo "Checking gofix..."
-go tool fix -diff $SRC
+# https://github.com/coreos/ignition/issues/1950
+go_version=$(go version | awk '{print $3}')
+go tool fix -go="$go_version" -diff $SRC
 
 echo "Checking gofmt..."
 res=$(gofmt -d -e -s $SRC)