Don't use a wildcard version requirement for schnorrkel dependency

[nathanwhit]

Description of proposed changes:
Glancing over our dependencies I noticed we weren't specifying a requirement, which makes it easy to accidentally get a breaking update when a new version is released.

Practical tips for PR review & merge:

• All GitHub Actions report PASS
• Newly added code/functions have unit tests
  • Coverage tools report all newly added lines as covered
  • The positive scenario is exercised
  • Negative scenarios are exercised, e.g. assert on all possible errors
  • Assert on events triggered if applicable
  • Assert on changes made to storage if applicable
• Modified behavior/functions - try to make sure above test items are covered
• Integration tests are added if applicable/needed

[github-actions]

For full LLVM coverage report click here!