all: test with the latest released Go version
all: test with the latest released Go version

This change updates the Makefile to call ./cmd/latestgo, which returns
the latest tagged version of Go. This is how CI also builds the
playground. Now, local testing is more similar.

Change-Id: I825ddb08573fade2c7451dcd44f97bd25c2253fa
Reviewed-on: https://go-review.googlesource.com/c/playground/+/297635
Trust: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/playground,golang/playground,golang/playground