Fix broken CI when git-merge needs to create a commit. (#11007)

- This fixes an error seen when CI needs to create a merge commit
   to sync the PR. CI started doing this recently to ensure that all
   CI branches use the same commit for their regressions.
 - The error looks like:

        + git merge e370ed4

        *** Please tell me who you are.

        Run

          git config --global user.email "you@example.com"
          git config --global user.name "Your Name"

        to set your account's default identity.
        Omit --global to set the identity only in this repository.

Jenkinsfile

@@ -45,7 +45,7 @@
 // 'python3 jenkins/generate.py'
 // Note: This timestamp is here to ensure that updates to the Jenkinsfile are
 // always rebased on main before merging:
-// Generated at 2022-04-13T15:32:37.844476
+// Generated at 2022-04-14T17:16:16.585491
 
 import org.jenkinsci.plugins.pipeline.modeldefinition.Utils
 // NOTE: these lines are scanned by docker/dev_common.sh. Please update the regex as needed. -->
@@ -121,7 +121,7 @@ def init_git() {
     ).trim()
   }
   sh (
-    script: "git merge ${upstream_revision}",
+    script: "git -c user.name=TVM-Jenkins -c user.email=jenkins@tvm.apache.org merge ${upstream_revision}",
     label: 'Merge to origin/main'
   )
 

jenkins/Jenkinsfile.j2

@@ -118,7 +118,7 @@ def init_git() {
     ).trim()
   }
   sh (
-    script: "git merge ${upstream_revision}",
+    script: "git -c user.name=TVM-Jenkins -c user.email=jenkins@tvm.apache.org merge ${upstream_revision}",
     label: 'Merge to origin/main'
   )