fix: ensure flake has all needed files

by not ignoring `.gitattributes`; without this file, Nix is apparently
liable to omit critical files (like `flake.nix`) from the paths copied
to the Nix store.

.gitattributes

@@ -11,13 +11,15 @@
 /vagrant/ export-ignore
 /vagrant/** export-ignore
 
-# XXX do not exclude the `/tests/` hierarchy, as doing so breaks GitHub
-# Actions.  That is, do not do this:
+# XXX do not exclude (a) the `/tests/` hierarchy, as doing so breaks GitHub
+# Actions, or (b) the `.gitattributes` file, as this appears to break the Nix
+# flake.
+# That is, do not do this:
 #   /tests/ export-ignore
 #   /tests/** export-ignore
+#   .gitattributes export-ignore
 
 *.nix export-ignore
-.gitattributes export-ignore
 .gitignore export-ignore
 .travis.yml export-ignore
 CONTRIBUTING.md export-ignore