cardano-tesnet: allow to specify output directory

[smelc]

Description

Adds the --output-dir to the cardano-testnet executable

Fixes #6078

How trust this PR

Use the new flag:

> ls /tmp > tmp-before # for checking data afterwards
> cabal run cardano-testnet -- cardano --testnet-magic 44 --output-dir output-dir

Witness that the output directory gets populated:

> tree output-dir
output-dir/
├── alonzo-genesis.json
├── byron-gen-command
│   └── genesis-keys.000.key
├── byron-genesis.json
├── byron.genesis.spec.json
├── configuration.yaml
├── conway-genesis.json
├── delegate-keys
│   ├── delegate1
│   │   ├── kes.skey
│   │   ├── kes.vkey
│   │   ├── key.skey
│   │   ├── key.vkey
...

You can check that the /tmp directory wasn't used, by comparing its current state with the state before:

> ls /tmp > tmp-after
> diff tmp-before tmp-after
# nothing gets printed

Now test that you can omit the flag:

> ls /tmp > tmp-before-2
> cabal run cardano-testnet -- cardano --testnet-magic 44

Observe that the testnet's log mentions a workspace in /tmp like before this PR:

    forAll77 =
      /tmp/nix-shell.hc4ID6/testnet-test-dc5d67c2dd8545dd
    
    forAll78 =
      Workspace: /tmp/nix-shell.hc4ID6/testnet-test-dc5d67c2dd8545dd

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff

[carbolymer]

My main remark is about reusing mkConf, the other ones a just styling suggestions.