Add Jenkins multibranch job to copy tags

Summary:
Currently tags are copied after a copybara trigger, which only happens
on pushes. Instead add a multibranch pipeline to Jenkins that also triggers
copybara. This will ensure that tags get copied over even without a commit
being pushed to the repo.

Test Plan: Run it in Jenkins

Reviewers: zasgar, michelle

Reviewed By: michelle
Signed-off-by: Copybara <copybara@pixielabs.ai>

Differential Revision: https://phab.corp.pixielabs.ai/D11649

GitOrigin-RevId: 15ba5c7abe473c64356451f47d16320c2a8c61fd

Jenkinsfile

@@ -124,6 +124,7 @@ isMainRun =  (env.JOB_NAME == 'pixie-main/build-and-test-all')
 isNightlyTestRegressionRun = (env.JOB_NAME == 'pixie-main/nightly-test-regression')
 
 isCopybaraPublic = env.JOB_NAME.startsWith('pixie-main/copybara-public')
+isCopybaraTags = env.JOB_NAME.startsWith('pixie-main/copybara-tags')
 isCopybaraPxAPI = env.JOB_NAME.startsWith('pixie-main/copybara-pxapi-go')
 
 isOSSMainRun = (env.JOB_NAME == 'pixie-oss/build-and-test-all')
@@ -1418,7 +1419,7 @@ if (isNightlyTestRegressionRun) {
   buildScriptForCloudProdRelease()
 } else if (isOSSCloudBuildRun) {
   buildScriptForOSSCloudRelease()
-} else if (isCopybaraPublic) {
+} else if (isCopybaraPublic || isCopybaraTags) {
   buildScriptForCopybaraPublic()
 } else if (isCopybaraPxAPI) {
   buildScriptForCopybaraPxAPI()