fix beginner setup/instructions for playground config

[zeroXbrock]

📝 Summary

• changes path of el_node_ipc_path in config-playground.toml to use $HOME/.playground/devnet instead of /tmp (aligns with current builder-playground defaults)
• adds a step in the README to replace '$HOME' with your actual home path
  • $HOME was not evaluating correctly for me (tested on my ubuntu machine), fully-qualified paths fixed it

💡 Motivation and Context

Following the instructions without changing anything gave me this error:

δ cargo run --bin rbuilder run config-playground.toml
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.67s
     Running `target/debug/rbuilder run config-playground.toml`
...(truncated)

Error: No such file or directory (os error 2)

Caused by:
    No such file or directory (os error 2)

Location:
    /home/dev/code/rbuilder/crates/rbuilder/src/live_builder/order_input/clean_orderpool.rs:30:20

The line of code given by the error connects to the IPC provider, which was being initialized with an invalid path given the current instructions & defaults.

✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[github-actions]

Benchmark results for e94c623

Report: https://flashbots-rbuilder-ci-stats.s3.us-east-2.amazonaws.com/benchmark/e94c623-112fb17/report/index.html

Date (UTC)	2024-10-30T17:55:53+00:00
Commit	e94c6230d8593c40a22e3ecd73f7eff91efc76e1
Base SHA	112fb17450f8e1b267fb0d38bad011830d92d1b9

Significant changes

None

[ryanschneider]

Interesting, for me the reth playground process uses /tmp/reth.ipc:

$ grep reth.ipc ~/.playground/devnet/logs/reth.log
2024-10-28T15:45:49.101095Z  INFO RPC IPC server started path=/tmp/reth.ipc

edit: Ah nevermind, this was before flashbots/builder-playground#27