26.0.1: cmake, disabled prebuilt, unable to build with vulkan option enabled

It was working in 26.0.0 and is new in 26.0.1. No matter which CMake options I set `-DVULKAN_USE_PRECOMPILED:BOOL=FALSE -DFSO_BUILD_WITH_VULKAN:BOOL=TRUE`, when enabling vulkan support it always tries to download stuff:
```
[...]
-- Downloading prebuilt libraries from "https://github.com/scp-fs2open/scp-prebuilt/releases/download/bin-b57636e0/bin-linux.tar.gz" (try 5/5)
-- Download of prebuilt binaries failed: "Could not resolve proxy name"!
-- Configuring incomplete, errors occurred!
[...]
CMake Error at lib/prebuilt.cmake:67 (message):
5 download attempts failed!
Call Stack (most recent call first):
lib/vulkan.cmake:96 (get_prebuilt_path)
lib/CMakeLists.txt:46 (include)
```

Removing the offending line `sed -e '/get_prebuilt_path/d' -i lib/vulkan.cmake` allows me to build, so it should probably be gated behind the `VULKAN_USE_PRECOMPILED` option and not prevent it from working as intended.

Complete build log: [fs2_open-26.0.1_build.log](https://github.com/user-attachments/files/32235057/fs2_open-26.0.1_build.log)