Update slackSend for ruby-2.6 builds

Closes: CCI-338
flag=none

Test Plan:
- Tested by faking out the logic to detect merge-patchsets,
otherwise this isn't really testable until we have future failures once
this is merged

Change-Id: I81a95dca425fc25e85a8a7425e864a0740cf1f49
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/234747
Reviewed-by: Rex Fleischer <rfleischer@instructure.com>
Reviewed-by: Jacob Powell <spowell@instructure.com>
Reviewed-by: James Butters <jbutters@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: James Butters <jbutters@instructure.com>
Product-Review: James Butters <jbutters@instructure.com>

Jenkinsfile

@@ -76,6 +76,10 @@ def isPatchsetPublishable() {
   env.PATCHSET_TAG == env.PUBLISHABLE_TAG
 }
 
+def isPatchsetSlackableOnFailure() {
+  env.SLACK_MESSAGE_ON_FAILURE == 'true' && env.GERRIT_EVENT_TYPE == 'change-merged'
+}
+
 // WARNING! total hack, being removed after covid...
 def isCovid() {
   env.GERRIT_BRANCH == 'covid'
@@ -395,7 +399,7 @@ pipeline {
   post {
     failure {
       script {
-        if (isPatchsetPublishable() && env.GERRIT_EVENT_TYPE == 'change-merged') {
+        if (isPatchsetSlackableOnFailure()) {
           def branchSegment = env.GERRIT_BRANCH ? "[$env.GERRIT_BRANCH]" : ''
           def authorSegment = env.GERRIT_EVENT_ACCOUNT_NAME ? "Patchset by ${env.GERRIT_EVENT_ACCOUNT_NAME}. " : ''
           slackSend(