Bump windows/nanoserver in /images (microsoft#1672)

Bumps windows/nanoserver from 10.0.20348.1129 to 10.0.20348.1249.

---
updated-dependencies:
- dependency-name: windows/nanoserver
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Alan Jowett <alanjo@microsoft.com>

images/Dockerfile.install

@@ -3,7 +3,7 @@
 
 # Base image for Windows nanoserver 1809.
 # The files in this image are copied to $env:CONTAINER_SANDBOX_MOUNT_POINT on the host.
-FROM mcr.microsoft.com/windows/nanoserver@sha256:fefb912e2942912f1cd55bfe6bbb697a601a7787caf5c1875aaf105c86a28027
+FROM mcr.microsoft.com/windows/nanoserver@sha256:f1125cf932664b039a55afb666bdca7dc28d453a88430fe47be763e0270638b6
 
 ENV PATH="C:\Program Files\PowerShell;C:\utils;C:\Windows\system32;C:\Windows;C:\Windows\System32\WindowsPowerShell\v1.0;"