FIX: COINCURVE_UPSTREAM_TAG -> COINCURVE_UPSTREAM_REF

[MementoRC]

I introduced that typo sometime back, it has no effect now, but with different version of SECP256K1, it will

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.68%. Comparing base (35b9f27) to head (7bc6765).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #155 +/- ## ======================================= Coverage 85.68% 85.68% ======================================= Files 11 11 Lines 566 566 Branches 67 67 ======================================= Hits 485 485 Misses 45 45 Partials 36 36 ```

[MementoRC]

A bit nervous