Allow to override build date

in builds from a release tarball, there is no git commit
to take a date from, so the current date was inserted there.

See https://reproducible-builds.org/ for why this matters
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This patch was done while working on reproducible builds for openSUSE.

utils/mkhtml.py

@@ -862,7 +862,12 @@ def to_title(name):
         is_addon=True if addon_path else False,
     )
     if git_commit["commit"] == "unknown":
-        date_tag = "Accessed: {date}".format(date=git_commit["date"])
+        sde = int(os.environ.get('SOURCE_DATE_EPOCH', 0))
+        if sde:
+            build_date = datetime.utcfromtimestamp(sde)
+        else:
+            build_date = git_commit["date"]
+        date_tag = "Accessed: {date}".format(date=build_date)
     else:
         date_tag = "Latest change: {date} in commit: {commit}".format(
             date=git_commit["date"], commit=git_commit["commit"]