Rename install-swift-nightly.ps1 to install-swift-nightly-main.ps1

This script is only used when Docker is enabled, and the workflow was using the wrong name, leading to build failures.

e.g. https://github.com/swiftlang/swift-build/actions/runs/19923980997/job/57119158460?pr=954

Easiest solution is to rename the file.

For Docker, both "nightly" and "nightly-main" tags exist, so we can just use "nightly-main" which is both more consistent with other platforms and easier to use across the board.

Author: jakepetroules

.github/workflows/scripts/windows/swift/install-swift-nightly.ps1 renamed to .github/workflows/scripts/windows/swift/install-swift-nightly-main.ps1

@@ -78,7 +78,7 @@ on:
         type: string
         description: "Include Windows Swift version list (JSON)"
         # "5.10" is omitted for Windows because the container image is broken.
-        default: "[\"5.9\", \"6.0\", \"6.1\", \"6.2\", \"nightly\", \"nightly-6.2\"]"
+        default: "[\"5.9\", \"6.0\", \"6.1\", \"6.2\", \"nightly-main\", \"nightly-6.2\"]"
       windows_exclude_swift_versions:
         type: string
         description: "Exclude Windows Swift version list (JSON)"