bci2: Add defaultBranch keyword argument

Passing the `defaultBranch` argument to `buildContainerImage2()` allows
setting the `latest` for projects where the default branch is not
*main*.  For example:

```groovy
buildContainerImage2(defaultBranch: 'master')
```

This will update the `latest` tag whenever a new build is created on the
*master* branch.

vars/buildContainerImage2.groovy

@@ -7,6 +7,7 @@ def call(args) {
     def tag = args?.tag
     def archlist = args?.archlist
     def schedule = args?.schedule
+    def defaultBranch = args?.defaultBranch
 
     properties([
         pipelineTriggers([cron(schedule ?: 'H H H * *')])
@@ -27,6 +28,9 @@ def call(args) {
     if (archlist == null) {
         archlist = ['amd64']
     }
+    if (defaultBranch == null) {
+        defaultBranch = 'main'
+    }
     def repo = "${registry}/${project}/${name}"
     def full_name = "${repo}:${tag}"
 
@@ -72,7 +76,7 @@ def call(args) {
                         }
                         sh "buildah push ${full_name} ${full_name}-${env.BUILD_NUMBER}"
                         sh "buildah push ${full_name}"
-                        if (env.BRANCH_NAME == 'main') {
+                        if (env.BRANCH_NAME == defaultBranch) {
                             sh "buildah push ${full_name} ${repo}:latest"
                         }
                     }