golang: v↑ go1.15 (1.15.5 -> 1.15.8)

Going Go1.15.5 -> 1.15.8 brings in fixes to compiler, runtime, standard library
and build security.

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

Tested on helloworld SR.

component/golang/buildout.cfg

@@ -61,8 +61,8 @@ environment-extra =
 
 [golang1.15]
 <= golang-common
-url = https://golang.org/dl/go1.15.5.src.tar.gz
-md5sum = 12a60d319ff58c03c323e8dcc125ae81
+url = https://golang.org/dl/go1.15.8.src.tar.gz
+md5sum = 7e4b8580bfe25ffc51501b1dc3d2d8ce
 
 # go1.15 needs go1.4 to bootstrap
 environment-extra =