Update to go 1.15.8

go1.15.8 (released 2021/02/04) includes fixes to the compiler, linker, runtime,
the go command, and the net/http package. See the Go 1.15.8 milestone on the
issue tracker for details.

https://github.com/golang/go/issues?q=milestone%3AGo1.15.8+label%3ACherryPickApproved

full diff: https://github.com/golang/go/compare/go1.15.7...go1.15.8

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

Vagrantfile

@@ -77,7 +77,7 @@ Vagrant.configure("2") do |config|
   config.vm.provision "install-golang", type: "shell", run: "once" do |sh|
     sh.upload_path = "/tmp/vagrant-install-golang"
     sh.env = {
-        'GO_VERSION': ENV['GO_VERSION'] || "1.15.7",
+        'GO_VERSION': ENV['GO_VERSION'] || "1.15.8",
     }
     sh.inline = <<~SHELL
         #!/usr/bin/env bash