Update Jenkinsfile (#4443)

Author: Ximinhan

jobs/build/gen-assembly/Jenkinsfile

@@ -37,6 +37,11 @@ node {
                             defaultValue: "https://github.com/openshift-eng/ocp-build-data",
                             trim: true,
                         ),
+                        string(
+                            name: "DATE",
+                            description: "(Optional) Leave empty to use auto-fetched release date. Intended release date. Format: YYYY-Mon-dd (example: 2050-Jan-01)",
+                            trim: true
+                        ),
                         string(
                             name: "NIGHTLIES",
                             description: "(Optional for public nightlies) List of nightlies to match with <code>doozer get-nightlies</code> (if empty, find latest). If preparing from private nightlies, provide the amd64 nightly as parameter, to match. The automation will find the corresponding nightlies for other arches.",
@@ -160,6 +165,9 @@ node {
                     cmd << "--nightly" << nightly.trim()
                 }
             }
+            if (params.DATE) {
+                cmd << "--date" << params.DATE
+            }
             if (params.ALLOW_PENDING) {
                 cmd << "--allow-pending"
             }