rebase: update golang version to v1.15.5

To fix math/big CVE, this version of golang has the fix
for this vulnerability.

Signed-off-by: Mudit Agarwal <muagarwa@redhat.com>

build.env

@@ -16,7 +16,7 @@ BASE_IMAGE=docker.io/ceph/ceph:v15
 CEPH_VERSION=nautilus
 
 # standard Golang options
-GOLANG_VERSION=1.15
+GOLANG_VERSION=1.15.5
 GO111MODULE=on
 
 # static checks and linters