[write-fonts/gvar] Fix shared tuples unstable order

[anthrotype]

Reproduce and fix googlefonts/fontc#647 (comment)

[anthrotype]

https://github.com/googlefonts/fontations/actions/runs/7528580955/job/20491085359?pr=756

failures:

---- tables::gvar::tests::shared_tuples_stable_order stdout ----
thread 'tables::gvar::tests::shared_tuples_stable_order' panicked at write-fonts/src/tables/gvar.rs:1270:13:
assertion `left == right` failed
  left: [[-1.0], [1.0]]
 right: [[1.0], [-1.0]]

[anthrotype]

I can confirm this enables reproducible builds for Oswald. I tested rebuilding a hundred times and fontc always produces exact same bytes for gvar, previously it would fail about 50% of the times.

[dfrg]

This looks like a reasonable fix to me, thanks!