Starts Noita from the given path, optionally with a specific seed set.

start-noita.ps1

function Start-Noita {
    <#
        .SYNOPSIS
        Starts Noita from a PowerShell prompt, optionally with a specific set seed for a new game.
        
        .DESCRIPTION
        To use this function, copy and paste the complete function definition into a PowerShell
        session, and then invoke it with `Start-Noita`. Supply `-Seed somevalue` if you would like
        to run a specific seed (see the examples below).
        
        .EXAMPLE
        PS> Start-Noita
        
        Starts Noita normally.
        
        .EXAMPLE
        PS> Start-Noita -Seed 1
        
        Starts Noita with a fixed seed value of 1.
        
        .EXAMPLE
        PS> Start-Noita -Seed 1 -Path C:\Noita\noita.exe
        
        Starts Noita from the requested location with a fixed seed value of 1.
        Use this form if your copy of Noita was not installed via Steam, or modify the default
        setting for $Path below before loading the function into your PowerShell session.
        
        .NOTES
        If you want, you can load this function into your PowerShell profile as follows:
        
        1. Copy the entire function definition from top to bottom to your Windows clipboard (Ctrl+C).
        2. Run the following code in PowerShell:
        
            Get-Clipboard | Add-Content -Path $profile
        
        3. Close and reopen PowerShell to load the newly modified profile.
        4. Run `Start-Noita` directly as needed. It will be automatically loaded in any future PowerShell sessions.
    #>
    [CmdletBinding(DefaultParameterSetName = 'Default')]
    param(
        [Parameter()]
        [string]
        $Path = "C:\Program Files (x86)\Steam\steamapps\common\Noita",

        [Parameter(Position = 0, ParameterSetName = 'Default')]
        [long]
        $Seed,

        [Parameter(ParameterSetName = 'Default')]
        [switch]
        $DevMode,

        [Parameter(Mandatory, ParameterSetName = 'Unpack')]
        [switch]
        $UnpackData
    )

    $noitaArgs = @()
    $noitaFolder = $Path | Split-Path -Parent

    if ($Seed) {
        $xml = [System.Xml.XmlDocument]::new()
        $node = $xml.CreateElement('MagicNumbers')
        $xml.AppendChild($node) > $null
        $blank = $xml.CreateTextNode([Environment]::NewLine)
        $node.AppendChild($blank) > $null
        $node.SetAttribute('WORLD_SEED', $Seed)
        $node.SetAttribute('_DEBUG_DONT_LOAD_OTHER_MAGIC_NUMBERS', 1)
        $node.SetAttribute('_DEBUG_DONT_SAVE_MAGIC_NUMBERS', 1)

        $magicPath = $Path | Join-Path -ChildPath 'magic.txt'
        $sb = [System.Text.StringBuilder]::new()
        $xmlWriter = [System.Xml.XmlWriter]::Create(
            $sb,
            @{
                Indent = $true
                NewLineOnAttributes = $true
                OmitXmlDeclaration = $true
            }
        )
        $xml.Save($xmlWriter)
        $xmlText = $sb.ToString()
        Write-Verbose $xmlText
        $xmlText | Set-Content -Path $magicPath

        $noitaArgs = @(
            '-no_logo_splashes'
            '-magic_numbers', 'magic.txt'
        )
    }
    elseif ($UnpackData) {
        $noitaArgs = @(
            '-wizard_unpak'
        )
    }

    $filename = if ($DevMode) {
        'noita_dev.exe'
    }
    else {
        'noita.exe'
    }

    $executable = Join-Path $Path -ChildPath $filename

    try {
        Push-Location $Path
        & $executable @noitaArgs
    }
    finally {
        Pop-Location
    }
}

I've tried editting magic.txt manually, then setting it as read only. The contents remain, instead of resetting to WORLD_SEED="0", etc.
Noita still loads a random seed though.