Include web export workflow in git archive

Since commit bb0bef1 we have included
the GitHub Actions workflow that exports the project for the web &
publishes it with GitHub Pages as part of the zip file created by `git
archive`, which is what is fetched when the project is downloaded via
the Godot Asset Library.

However in commit 80440de I split that
workflow in two, without updating `.gitattributes` accordingly. As a
result, the "Publish to GitHub Pages" workflow is present, but the
"Build and Export Game" workflow is not; and the former (understandably)
fails.

Include it.

Author: wjt

.gitattributes

@@ -1,11 +1,13 @@
 # Normalize EOL for all files that Git considers text files.
 * text=auto eol=lf
 
-# Exclude GitHub Actions-related files from published asset, except the GitHub
-# Pages workflow which learners may find useful to publish their modified game.
-/.github/**                           export-ignore
+# Exclude GitHub Actions-related files from published asset, except the web
+# export & GitHub Pages workflows which learners may find useful to publish
+# their modified game.
+/.github/** export-ignore
 /.github/workflows                   !export-ignore
 /.github/workflows/**                 export-ignore
+/.github/workflows/export.yml        !export-ignore
 /.github/workflows/github-pages.yml  !export-ignore
 /asset-template.json.hb               export-ignore