Fix Stable Builds (#39)

Stable builds key off of `IS_STABLE` to resolve which image to pull into
the RPM. The `IS_STABLE` env. var. was removed in a previous PR.

This adds `IS_STABLE` back in to allow stable builds to work again.

Jenkinsfile.github

@@ -1,18 +1,18 @@
 /*
  MIT License
- 
+
  (C) Copyright 2022 Hewlett Packard Enterprise Development LP
- 
+
  Permission is hereby granted, free of charge, to any person obtaining a
  copy of this software and associated documentation files (the "Software"),
  to deal in the Software without restriction, including without limitation
  the rights to use, copy, modify, merge, publish, distribute, sublicense,
  and/or sell copies of the Software, and to permit persons to whom the
  Software is furnished to do so, subject to the following conditions:
- 
+
  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.
- 
+
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
@@ -45,6 +45,7 @@ pipeline {
     environment {
         DOCKER_ARGS = getDockerBuildArgs(name: getRepoName(), description: 'A cloud-init DataSource.')
         NAME = getRepoName()
+        IS_STABLE = "${isStable}"
         GO_VERSION = sh(returnStdout: true, script: 'grep -Eo "^go .*" go.mod | cut -d " " -f2').trim()
         IMAGE_VERSION = sh(returnStdout: true, script: "git describe --tags | tr -s '-' '_' | tr -d '^v'").trim()
         VERSION = sh(returnStdout: true, script: "git describe --tags | tr -s '-' '~' | tr -d '^v'").trim()