nonspec: add zachariahcox to maintainers group (#1173)

I'd like to propose making myself a maintainer, initially scoped to the
source-track if needed!

I believe I meet the requirements [listed
here](https://github.com/slsa-framework/slsa/blob/main/MAINTAINERS.md#becoming-a-maintainer).

Signed-off-by: Zachariah Cox <zachariahcox@github.com>

MAINTAINERS.md

@@ -23,6 +23,7 @@ permissions in this repository.
 | Mark Lodato | lodato@google.com |  @Mark Lodato | [MarkLodato](https://github.com/MarkLodato) | Google
 | Michael Lieberman | mlieberman85@gmail.com | @Michael Lieberman | [mlieberman85](https://github.com/mlieberman85) | Kusari
 | Tom Hennen | tomhennen@google.com | @Tom Hennen | [TomHennen](https://github.com/TomHennen) | Google
+| Zachariah Cox | zachariahcox@github.com | @zachariahcox | [zachariahcox](https://github.com/zachariahcox) | GitHub
 
 ### Becoming a Maintainer