fix: never reuse a backup folder of another run

When every dated name in retrobios-backup already existed, the loop ended on the last one and wrote into it. It now raises, so the next location is tried and the file stays in place if none works. A holder another user owns is refused on systems with user ids.

install.ps1

@@ -11,7 +11,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "001ddf03177f12051e70d5f57db788316c7971f23366ff2532f91a2b9aca2ffd"
+    $defaultInstallSha256 = "c595032e68eec7573ad418cac8b906b0fc84a2253367f02e480d21eef94e1980"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }