ci: add CSI_UPGRADE_VERSION var to build.env

Currently, upgrade version for upgrade tests
need to be set in ci/centos branch.
This commit adds a variable in build.env,
so that we have the flexibility to use
this value instead.

> Backport of: #4008

Signed-off-by: Rakshith R <rar@redhat.com>

build.env

@@ -11,6 +11,9 @@
 # cephcsi image version
 CSI_IMAGE_VERSION=v3.9-canary
 
+# cephcsi upgrade version
+CSI_UPGRADE_VERSION=v3.8.1
+
 # Ceph version to use
 BASE_IMAGE=quay.io/ceph/ceph:v17
 CEPH_VERSION=quincy