Move checkout inside try block

Prior to this commit the checkout call was outside the try block. This commit
fixes that by moving the checkout into the try block incase it fails and the
working directory still needs to be cleaned up.

Jenkinsfile

@@ -17,9 +17,9 @@ node {
         def platform = platforms[platform_idx]
         builds[platform] = {
             node {
-                checkout scm
-
                 try {
+                    checkout scm
+
                     image = docker.build("cmake-build:${env.BUILD_ID}")
 
                     stage("Build ${platform}") {