CI: Remove macos-15 builder

Fails at the "Install Nix"-step with:

```
  ~~> Setting up the build user _nixbld1
  <main> attribute status: eDSRecordAlreadyExists
  <dscl_cmd> DS Error: -14135 (eDSRecordAlreadyExists)
```

Author: runeksvendsen

.github/workflows/cabal-in-nix-shell.yml

@@ -8,7 +8,7 @@ jobs:
   build_init:
     strategy:
       matrix:
-        os: [ubuntu-22.04, ubuntu-20.04, macos-13, macos-14, macos-15]
+        os: [ubuntu-22.04, ubuntu-20.04, macos-13, macos-14] # TODO: add macos-15 once "Install Nix"-step works
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps:

.github/workflows/nix-build.yml

@@ -8,7 +8,7 @@ jobs:
   nix-build:
     strategy:
       matrix:
-        os: [ubuntu-22.04, ubuntu-20.04, macos-13, macos-14, macos-15]
+        os: [ubuntu-22.04, ubuntu-20.04, macos-13, macos-14] # TODO: add macos-15 once "Install Nix"-step works
     runs-on: ${{ matrix.os }}
     permissions:
       contents: read