fix(flake): include *.nix files in git archive

by not specifying `*.nix export-ignore` in `.gitattributes`.  Hopefully
this fixes this error for good:

    error: getting status of '/nix/store/kd87l94qzrkjrh91mrymdcqzllikslwj-source/flake.nix': No such file or directory

Where previous attempts (like removing `.gitattributes export-ignore`)
failed.

Oddly, it seems that changing *any* `export-ignore` directive in
`.gitattributes` "fixes" the problem, but only temporarily.  However,
since the error above complains specifically about a `*.nix` file, and
because all files not exluded in `.gitignore` or marked with
`export-ignore` in `.gitattributes` appear in the flake store path
(above, `/nix/store/kd87l94qzrkjrh91mrymdcqzllikslwj-source/flake.nix`),
it seems like un-ignoring all `*.nix` files ought to correct the
issue.

.gitattributes

@@ -12,14 +12,13 @@
 /vagrant/** export-ignore
 
 # 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.
+# Actions, or (b) `*.nix` files, 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
 
-*.nix export-ignore
+.gitattributes export-ignore
 .gitignore export-ignore
 .travis.yml export-ignore
 CONTRIBUTING.md export-ignore