CMake build error on NixOS
Hello,

When I attempt to build p2pool using the following command:
```
nix run git+https://github.com/SChernykh/p2pool?ref=master -- --help
```
I get the following error:
```
error: builder for '/nix/store/2dwhn0f5xpnzbypv1w9l9djqsmdaz84i-p2pool-0.0.1.drv' failed with exit code 1;
       last 10 log lines:
       > CMake Error at CMakeLists.txt:12 (add_subdirectory):
       >   The source directory
       >
       >     /build/source/external/src/RandomX
       >
       >   does not contain a CMakeLists.txt file.
       >
       >
       > -- Configuring incomplete, errors occurred!
       > See also "/build/source/build/CMakeFiles/CMakeOutput.log".
       For full logs, run 'nix log /nix/store/2dwhn0f5xpnzbypv1w9l9djqsmdaz84i-p2pool-0.0.1.drv'.
```

Is there anything I am forgetting to do?
I know that there is a merge request for p2pool in the pipeline but I'm not sure when that will be complete.

NixOS Version: 21.11pre333218.263ef4cc414 (Porcupine)
