libaom: 3.0.0 -> 3.1.0
libaom: 3.0.0 -> 3.1.0

We had to use fetchgit so far as the tarballs are generated on demand
and have embedded timestamps which makes their hashes unstable [0][1].
This is a problem for fetchurl but fetchzip extracts the tarballs into
the Nix store and therefore the contents will get normalized and the
hashes remain stable.

[0]: https://github.com/google/gitiles/issues/84
[1]: https://bugs.eclipse.org/bugs/show_bug.cgi?id=548312

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