CI: Use x64 not arm64 macOS

ARM (M1) GitHub runner doesn't have Python 3.8 available. EOL of 3.8 is
31 Oct 2024, would be good to continue support, unless there are some
bigger reasons to drop it sooner.

And macos-latest was recently changed to M1, see
<https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories>.

This fixes current non-random always happening CI failures.

.github/workflows/unittests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-latest, ubuntu-latest]
+        os: [macos-13, ubuntu-latest]
         python-version: ["3.8", "3.11"]
         bitcoind-version: ["0.18.0", "26.0"]