Jenkins: T2625: fix proper library version used by build pipeline

Jenkinsfile

@@ -17,7 +17,7 @@
 // Using a version specifier library, use 'current' branch. The underscore (_)
 // is not a typo! You need this underscore if the line immediately after the
 // @Library annotation is not an import statement!
-@Library('vyos-build@current')_
+@Library('vyos-build@sagitta')_
 
 // Start package build using library function from https://github.com/vyos/vyos-build
 buildPackage(null, null, null, true)