fix: prevent duplicate --datadir argument in reth module (#640)

The reth module was adding --datadir twice: once manually in the script
and once through filtered args processing. This caused reth to fail with
"argument '--datadir' cannot be used multiple times".

Fixed by adding --datadir to specialArgs list to filter it out from
automatic processing, since it's already handled manually in the script.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: shekhirin

modules/reth/default.nix

@@ -82,6 +82,7 @@ in {
 
               # filter out certain args which need to be treated differently
               specialArgs = [
+                "--datadir"
                 "--authrpc.jwtsecret"
                 "--http.enable"
                 "--metrics.enable"