Prevent the extension manifests from embedding the current date.

This is inspired by the Debian
"Use-build-id-as-langpack-version-for-reproducibility.patch" maintained for
their Firefox package as well as reproducible-builds.org guidelines from
https://reproducible-builds.org/docs/source-date-epoch/.

--- ./python/mozbuild/mozbuild/action/langpack_manifest.py.old	2023-02-13 11:01:21.051537347 -0500
+++ ./python/mozbuild/mozbuild/action/langpack_manifest.py	2023-02-13 11:04:33.141817330 -0500
@@ -36,6 +36,15 @@
 pushlog_api_url = "{0}/json-rev/{1}"
 
 
+import os
+import time
+import datetime
+
+def get_build_date():
+    """Honor SOURCE_DATE_EPOCH for reproducibility."""
+    return datetime.datetime.utcfromtimestamp(
+        int(os.environ.get('SOURCE_DATE_EPOCH', time.time())))
+
 ###
 # Retrievers a UTC datetime of the push for the current commit
 # from a mercurial clone directory.
@@ -54,7 +63,7 @@
     with mozversioncontrol.get_repository_object(path=path) as repo:
         phase = repo._run("log", "-r", ".", "-T" "{phase}")
         if phase.strip() != "public":
-            return datetime.datetime.utcnow()
+            return get_build_date()
         repo_url = repo._run("paths", "default")
         repo_url = repo_url.strip().replace("ssh://", "https://")
         repo_url = repo_url.replace("hg://", "https://")
@@ -105,7 +114,7 @@
         dt = get_dt_from_hg(path)
 
     if dt is None:
-        dt = datetime.datetime.utcnow()
+        dt = get_build_date()
 
     dt = dt.replace(microsecond=0)
     return dt.strftime("%Y%m%d%H%M%S")