Remove redundant Get-AzEnvironment calls in custom environment support

[Copilot]

PR Azure#29067 calls Get-AzEnvironment twice in the same code path: once in the conditional check, again to access properties.

Changes

• Store Get-AzEnvironment result in $azEnv variable before use
• Apply to 4 functions: $registerArcScript, Get-ManagementUrl, Get-PortalDomain, Get-EnvironmentEndpoints
• Update ChangeLog description

Example

Before:

elseif ($null -ne (Get-AzEnvironment -Name $EnvironmentName))
{
    $managementUrl = (Get-AzEnvironment -Name $EnvironmentName).ResourceManagerUrl
}

After:

else
{
    $azEnv = Get-AzEnvironment -Name $EnvironmentName
    if ($null -ne $azEnv)
    {
        $managementUrl = $azEnv.ResourceManagerUrl
    }
}

Reduces 8 calls to 4 across the affected code paths.

Original prompt

Remove the duplication of fetching azure environment again and again in the changed code in pull request Azure#29067

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