Fixes a bug where the build system tried to call `git` to embed the
release tag revision into the binary, which isn't allowed. Easily fixed
with `substituteInPlace`.
Also updates the expression to be a little closer to "standard"
expressions (e.g. consistent indentation, importing `stdenv` instead of
`lib` and `mkDerivation`, etc.)
Signed-off-by: Austin Seipp <aseipp@pobox.com>