component/golang: Add version 1.15 needed for gitlab 13

component/golang/buildout.cfg

@@ -89,6 +89,15 @@ patches +=
   https://lab.nexedi.com/kirr/go/commit/6dfc2256e2cd1ad46f3a5e9da85af6a70be7fba3.patch#fa9de83838b198490abec0a6a924ad8d
   https://lab.nexedi.com/kirr/go/commit/28fbdd01d6c89db1e81e12bd05910fdec4c3b9b3.patch#c75e2eb7aaaeffdd3c507ec1beacd3ca
 
+[golang1.15]
+<= golang-common-pre-1.19
+url = https://go.dev/dl/go1.15.15.src.tar.gz
+md5sum = 05fedd8289291eb2d91cd0c092b41aaa
+
+# go1.16 needs go1.4 to bootstrap
+environment-extra =
+  GOROOT_BOOTSTRAP=${golang14:location}
+
 [golang1.16]
 <= golang-common-pre-1.19
 url = https://golang.org/dl/go1.16.13.src.tar.gz