golang: v↑ go1.15 (1.15.11 -> 1.15.12)

Going Go1.15.11 -> Go1.15.12 brings in fixes to compiler, runtime and stdlib
including security fix to net/http package:

https://golang.org/doc/devel/release.html#go1.15.minor

Tested on helloworld SR (by adjusting it to use go1.15 instead of go1.16).

component/golang/buildout.cfg

@@ -44,8 +44,8 @@ environment-extra =
 
 [golang1.15]
 <= golang-common
-url = https://golang.org/dl/go1.15.11.src.tar.gz
-md5sum = bd7617d2b959eb6e0b0df2a19e435a06
+url = https://golang.org/dl/go1.15.12.src.tar.gz
+md5sum = b3a5dafa7028fa00083ce4aeaf5bcb9a
 
 # go1.15 needs go1.4 to bootstrap
 environment-extra =