feat: Adjust GitHub Actions workflow to include macOS 15 and update environment variables for user site installation

Update `nest_vars.sh.in` and `nest-config.in` to set paths using 
`site.USER_SITE` for user-specific installations. Modify `slistartup.cc` 
and `module_manager.cpp` to utilize the new `SLI_PATH` environment 
variable. Adjust GitHub Actions workflow to include macOS 15 in the 
build matrix and clean up commented-out code.

Author: steffengraber

.github/workflows/build_nest_wheel.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-13, macos-14, ubuntu-24.04]        
+        os: [macos-13, macos-14, macos-15, ubuntu-24.04]        
     steps:
       - uses: actions/checkout@v4
 
@@ -18,10 +18,7 @@ jobs:
           package-dir: .
           output-dir: wheelhouse
           config-file: "{package}/pyproject.toml"
-        # env:
-          # CIBW_ENVIRONMENT_MACOS: 
-            # LDFLAGS="-L/usr/local/opt/readline/lib -L/usr/local/opt/ncurses/lib"
-            # CPPFLAGS="-I/usr/local/opt/readline/include -I/usr/local/opt/ncurses/include"
+    
       - uses: actions/upload-artifact@v4
         with:
           name: cibw-wheels-${{ matrix.os }}-${{ strategy.job-index }}

build_support/module_manager.cpp.patch

@@ -81,7 +81,6 @@ with-hdf5 = "OFF"
 [tool.cibuildwheel]
 build-frontend = "default"
 build = ["cp39-*", "cp310-*", "cp311-*", "cp312-*"]
-# build = ["cp312-*"]
 skip = "*-win* *-manylinux_i686" # Skip
 test-skip = ""
 free-threaded-support = false