Remove 'set -u' parameter because staging script is failing on an unbound var

Staging for releases is failing on an unbound variable. This is because there
is no suffix to append in the new Spring version format.

/opt/concourse-java.sh: line 81: $2: unbound variable

ci/scripts/stage.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-set -euo pipefail
+set -eo pipefail
 
 # shellcheck source=scripts/common.sh
 source "$(dirname "$0")/common.sh"