Remove Mac OS X from CI matrix due to issues with flaky cargo resolution (#2)

Removing Mac OS X from the CI matrix because:
- There are intermittent failures resolving "cargo fmt" that don't occur on ubuntu
- Several fix attempts have not worked
- Development is done on Mac, so having CI crosscheck linux is sufficient
- Mac runners are relatively expensive for the value they provide

Author: tfenne

.github/CODEOWNERS

@@ -1,2 +1 @@
-# Default owners: Nils and Tim
-*  @nh13 @tfenne
+*  @tfenne

.github/workflows/build_and_test.yml

@@ -14,7 +14,10 @@ jobs:
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable, 2026-02-12
+        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable, 2026-03-27
+        with:
+          toolchain: "1.95"
+          components: rustfmt, clippy
 
       - name: Cache dependencies
         uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
@@ -27,17 +30,16 @@ jobs:
 
   precommit:
     name: Pre-commit
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [ubuntu-24.04, macOS-latest]
+    runs-on: ubuntu-24.04
     steps:
       - name: Checkout sources
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable, 2026-02-12
+        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable, 2026-03-27
+        with:
+          toolchain: "1.95"
+          components: rustfmt, clippy
 
       - name: Cache dependencies
         uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2