fix(simapp): read file config and then flags

[facundomedica]

Description

Closes: #17822

I don't know how to test this, sadly. Ideas appreciated

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• run make lint and make test
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[odeke-em]

@facundomedica we can do the following in the unit test

• Grab the original value of the DefaultNodeHome
• Set a directory inside testing.T.TempDir()
• Create a new directory then write the files in there

func TestReadConfigThenFlags(t *testing.T) {
    origDefaultNodeHome := simapp.DefaultNodeHome
    defer func() {
        simapp.DefaultNodeHome = origDefaultNodeHome
    }()

    // 1. Generate the new node home
    newNodeHome := filepath.Join(t.TempDir(), "simapp")
    simapp.DefaultNodeHome = newNodeHome
    // 2. Write a config in there
    configFile := filepath.Join(newNodeHome, "config.json")
    if err := os.WriteFile(configFile, configJSON, 0644); err != nil {
         t.Fatal(err)
    }

[julienrbrt]

Taking this over in #17920.
Cannot reproduce flags issues, but we're discussing on Slack about it.