Pull request: scripts: imp sh scripts

Updates #2275.

Squashed commit of the following:

commit 2c44e7f490eafcfb2798037f9e0b7e6f8af42b75
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri May 21 15:32:44 2021 +0300

    scripts: imp sh scripts

scripts/make/go-build.sh

@@ -35,7 +35,8 @@ readonly x_flags v_flags
 # expansion (-f), and consider undefined variables as errors (-u).
 set -e -f -u
 
-# Allow users to set the Go version.
+# Allow users to override the go command from environment.  For example, to
+# build two releases with two different Go versions and test the difference.
 go="${GO:-go}"
 readonly go