website: Stop mike from always changing sitemaps.xml.gz

Originally, my motivation was to try again to get `mike` to not push empty
commits (which this should do). I'm now reconsidering this, since *not* pushing
empty commits will make the output of the CI job a little harder to read. If
this becomes an issue, I  might even add `--allow-empty` to the `mike`
invocations later.

A more important motivation is that even for a 400-byte file, changing it for
every PR blows up the size of the repo eventually.

The cause for the changes to this file was that `gzip` stores a timestamp
inside the `.gz` file.

.github/scripts/docs-build-deploy

@@ -9,6 +9,10 @@
 set -ev
 
 export "SITE_URL_FOR_MKDOCS=$1"; shift
+# Affects the generation of `sitemap.xml.gz` by `mkdocs`. See
+# https://github.com/jimporter/mike/issues/103 and
+# https://reproducible-builds.org/docs/source-date-epoch/
+export SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct docs/ mkdocs.yml)
 # https://github.com/python-poetry/poetry/issues/1917 and
 # https://github.com/python-poetry/poetry/issues/8623
 export PYTHON_KEYRING_BACKEND=keyring.backends.fail.Keyring