Update gen_windows.ps1

Fixed improper env references

llm/generate/gen_windows.ps1

@@ -243,9 +243,9 @@ if ($null -ne $script:CUDA_LIB_DIR) {
     init_vars
     $script:buildDir="../build/windows/${script:ARCH}/cuda$script:CUDA_VARIANT"
     $script:cmakeDefs += @("-A", "x64", "-DLLAMA_CUDA=ON", "-DLLAMA_AVX=on", "-DLLAMA_AVX2=off", "-DCUDAToolkit_INCLUDE_DIR=$script:CUDA_INCLUDE_DIR", "-DCMAKE_CUDA_ARCHITECTURES=${script:CMAKE_CUDA_ARCHITECTURES}")
-    if ($null -ne $script:OLLAMA_CUSTOM_CUDA_DEFS) {
-        write-host "OLLAMA_CUSTOM_CUDA_DEFS=`"${script:OLLAMA_CUSTOM_CUDA_DEFS}`""
-        $script:cmakeDefs +=@("${script:OLLAMA_CUSTOM_CUDA_DEFS}")
+    if ($null -ne $env:OLLAMA_CUSTOM_CUDA_DEFS) {
+        write-host "OLLAMA_CUSTOM_CUDA_DEFS=`"${env:OLLAMA_CUSTOM_CUDA_DEFS}`""
+        $script:cmakeDefs +=@("${env:OLLAMA_CUSTOM_CUDA_DEFS}")
         write-host "building custom CUDA GPU"
     }
     build
@@ -279,9 +279,9 @@ if ($null -ne $env:HIP_PATH) {
 
     # We have to clobber the LIB var from the developer shell for clang to work properly
     $env:LIB=""
-    if ($null -ne $script:OLLAMA_CUSTOM_ROCM_DEFS) {
-        write-host "OLLAMA_CUSTOM_ROCM_DEFS=`"${script:OLLAMA_CUSTOM_ROCM_DEFS}`""
-        $script:cmakeDefs += @("${script:OLLAMA_CUSTOM_ROCM_DEFS}")
+    if ($null -ne $env:OLLAMA_CUSTOM_ROCM_DEFS) {
+        write-host "OLLAMA_CUSTOM_ROCM_DEFS=`"${env:OLLAMA_CUSTOM_ROCM_DEFS}`""
+        $script:cmakeDefs += @("${env:OLLAMA_CUSTOM_ROCM_DEFS}")
         write-host "building custom ROCM GPU"
     }
     write-host "Building ROCm"