dotnet-sdk-setup-hook: use temporary symlink dir for NUGET_FALLBACK_PACKAGES

Restore operations get extremely slow when there are a lot of paths in
NUGET_FALLBACK_PACKAGES.

pkgs/development/compilers/dotnet/dotnet-sdk-setup-hook.sh

@@ -28,8 +28,9 @@ _linkPackages() {
 createNugetDirs() {
     nugetTemp=$PWD/.nuget-temp
     export NUGET_PACKAGES=$nugetTemp/packages
+    export NUGET_FALLBACK_PACKAGES=$nugetTemp/fallback
     nugetSource=$nugetTemp/source
-    mkdir -p "$NUGET_PACKAGES" "$nugetSource"
+    mkdir -p "$NUGET_PACKAGES" "$NUGET_FALLBACK_PACKAGES" "$nugetSource"
 
     dotnet new nugetconfig
     if [[ -z ${keepNugetConfig-} ]]; then
@@ -44,7 +45,7 @@ configureNuget() {
 
     for x in "${!_nugetInputs[@]}"; do
         if [[ -d $x/share/nuget/packages ]]; then
-            addToSearchPathWithCustomDelimiter ";" NUGET_FALLBACK_PACKAGES "$x/share/nuget/packages"
+            _linkPackages "$x/share/nuget/packages" "$NUGET_FALLBACK_PACKAGES"
         fi
 
         if [[ -d $x/share/nuget/source ]]; then