dotnet: don't set $HOME if its already writable

This fixes an issue where the setup hook would overwrite $HOME in a
nix-shell.

Fixes #164450, #164295

pkgs/development/compilers/dotnet/build-dotnet.nix

@@ -90,7 +90,10 @@ in stdenv.mkDerivation rec {
   '';
 
   setupHook = writeText "dotnet-setup-hook" ''
-    export HOME=$(mktemp -d) # Dotnet expects a writable home directory for its configuration files
+    if [ ! -w "$HOME" ]; then
+      export HOME=$(mktemp -d) # Dotnet expects a writable home directory for its configuration files
+    fi
+
     export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1 # Dont try to expand NuGetFallbackFolder to disk
     export DOTNET_NOLOGO=1 # Disables the welcome message
     export DOTNET_CLI_TELEMETRY_OPTOUT=1