`SHADERC_SKIP_INSTALL` cmake option broken

Running cmake with the `SHADERC_SKIP_INSTALL` option will break cmake:

```
...
**CMake Error: install(EXPORT "glslang-targets" ...) includes target "SPIRV" which requires target "SPIRV-Tools-opt" that is not in any export set.**
-- Generating done
CMake Generate step failed. Build files cannot be regenerated correctly.
```
I think the issue is https://github.com/google/shaderc/blob/main/third_party/CMakeLists.txt#L90:
```
set(GLSLANG_ENABLE_INSTALL $)
```
I don't believe that "$