ci(windows): Skip installing Inno Setup if already installed

GitHub had removed Inno Setup from the Windows Server 2025 image, but
now they've recently reintroduced it:

https://redirect.github.com/actions/runner-images/issues/12947#issuecomment-3416540769

This new image with Inno Setup pre-installed is not generally available
yet, so we still need to keep the "Install latest Inno Setup" CI step
around, because our Windows CI runner might be randomly assigned an
older version of the Windows 2025 image, where Inno Setup doesn't come
pre-installed.

To prevent that step from randomly failing, however, we now check if
Inno Setup is already installed before invoking "winget install".

Author: guihkx

.github/workflows/windows.yml

@@ -161,9 +161,12 @@ jobs:
 
       - name: Install latest Inno Setup
         run: |
-          winget install --id JRSoftware.InnoSetup --exact --silent --source winget
-          # Add to PATH
-          Add-Content $env:GITHUB_PATH "$env:LOCALAPPDATA\Programs\Inno Setup 6"
+          # Only installs if Inno Setup's Compiler (ISCC.exe) is not already in the PATH environment variable.
+          if (-Not (Get-Command ISCC -ErrorAction SilentlyContinue)) {
+              winget install --id JRSoftware.InnoSetup --exact --silent --source winget
+              # Add to PATH
+              Add-Content $env:GITHUB_PATH "$env:LOCALAPPDATA\Programs\Inno Setup 6"
+          }
 
       - name: Create installer
         run: |