Fix CI Issues

[ni-mheppner]

• Pin libflac 1.3.4, as newer versions introduce changes breaking our tests
  • This is to revive the CI pipeline - the underlying issue still needs to be looked into
• Fix Xcode version for Macos 12 builds

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f81cd09) 83.10% compared to head (a66be7e) 83.10%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #60 +/- ## ======================================= Coverage 83.10% 83.10% ======================================= Files 82 82 Lines 2024 2024 ======================================= Hits 1682 1682 Misses 342 342 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NativeInstruments). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NativeInstruments)

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