Use internal variable to check for vim-dispatch

CMakeCreateBuild will fail without vim-dispatch installed with the following error. It seems it was set up to fix this by setting g:cmake_use_dispatch on load, but the code didn't use the internal variable.

```
[cmake] Configuring project for the first time...
Error detected while processing function cmake#commands#create_build..cmake#util
#run_cmake..cmake#util#shell_exec:
line    1:
E121: Undefined variable: g:loaded_dispatch
E15: Invalid expression: g:loaded_dispatch == 1
```