ci(gitlab,windows): preserve exclusions during dependency setup

Creating .git/info/exclude as a file with `New-Item` and `-Force`
truncates existing contents.

When install-dependencies.ps1 follows install-sdk.ps1, this discards
the latter's /git-sdk exclusion and causes ci/lib.sh to reject SDK
files as unignored build artifacts.

Assisted-by: GPT-6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/install-dependencies.ps1

@@ -12,7 +12,9 @@ $RustVersion = '1.96.0'
 
 New-Item -Path $DownloadDirectory -ItemType Directory -Force | Out-Null
 New-Item -Path .git/info -ItemType Directory -Force | Out-Null
-New-Item -Path .git/info/exclude -ItemType File -Force | Out-Null
+if (-not (Test-Path .git/info/exclude)) {
+    New-Item -Path .git/info/exclude -ItemType File | Out-Null
+}
 Add-Content -Path .git/info/exclude -Value "/$DownloadDirectory"
 
 function Get-Installer {