fix: update Miniconda download method and add error handling

[LummiGhost]

use iwr ( Invoke-WebRequest ) instead of Net.WebClient. Fix 403 error.

         powershell -Command "(New-Object Net.WebClient).DownloadFile('https://repo.anaconda.com/miniconda/Miniconda3-py310_24.9.2-0-Windows-x86_64.exe', '.\envs\miniconda3.exe')"                                  
╰─❯                                                                                            
Exception calling "DownloadFile" with "2" argument(s): "The remote server returned an error: (403) Forbidd
en."
At line:1 char:1
+ (New-Object Net.WebClient).DownloadFile('https://repo.anaconda.com/mi ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : WebException

[Copilot]

Pull request overview

This PR fixes a 403 Forbidden error encountered when downloading Miniconda by replacing the deprecated Net.WebClient method with PowerShell's Invoke-WebRequest (iwr) command. It also adds error handling to verify successful download before proceeding with installation.

Key Changes:

• Replaced Net.WebClient.DownloadFile() with iwr command for downloading Miniconda installer
• Added validation to check if the installer file exists after download, with proper error messaging and exit handling
• Updated .gitignore to exclude envs.back/ directory

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated no comments.

File	Description
ezvtuber-rt	Updated subproject commit reference
.gitignore	Added envs.back/ to ignored directories
	Replaced download method and added error handling for Miniconda installation

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[LummiGhost]

WTF, 我不是撤掉了第二个提交吗？