Fix Firefox install failing when msstore source returns certificate error

Add --source winget to winget install call so the package is resolved
only from the winget source, avoiding the msstore TLS failure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

powershell/Install-Firefox.ps1

@@ -97,6 +97,7 @@ function Install-FirefoxEsr {
         $Arguments = @(
             "install",
             "--id", $PackageId,
+            "--source", "winget",
             "--silent",
             "--accept-source-agreements",
             "--accept-package-agreements"

powershell/libres-softwares-install.ps1

@@ -93,6 +93,7 @@ function Install-FirefoxEsr {
         $Arguments = @(
             "install",
             "--id", $PackageId,
+            "--source", "winget",
             "--silent",
             "--accept-source-agreements",
             "--accept-package-agreements"