assert submodules are available to Nix derivation

[jakubgs]

This should result in a human-readable error when ?submodules=1# is not appended to the project URL as the README explains:

 > nix build '.'
       error: Unable to build without submodules. Append '?submodules=1#' to the URL.

 > nix build '.?submodules=1#'

[tersec]

Why would this error condition happen to begin with?

[github-actions]

Unit Test Results

       12 files  ±0    1 822 suites  ±0   57m 53s ⏱️ - 1m 35s
  5 347 tests ±0    5 000 ✔️ ±0  347 💤 ±0  0 ❌ ±0 
29 565 runs  ±0  29 121 ✔️ ±0  444 💤 ±0  0 ❌ ±0 

Results for commit 9ccd1c7. ± Comparison against base commit c63862c.

[jakubgs]

Because if we do not specify ?submodules=1# in the URL when building the node via Nix then submodules will not be present and the build will fail. In a perfect would this would be configurable in the flake.nix itself, but currently it is not. For more context on problems with submodules you can read this issue:

• Submodules of flakes are not working NixOS/nix#6633

I've added this assert because without it the error is incomprehensible to a mere mortal.

[tersec]

Because if we do not specify ?submodules=1# in the URL when building the node via Nix then submodules will not be present and the build will fail. In a perfect would this would be configurable in the flake.nix itself, but currently it is not. For more context on problems with submodules you can read this issue:

* [Submodules of flakes are not working NixOS/nix#6633](https://github.com/NixOS/nix/issues/6633)

I've added this assert because without it the error is incomprehensible to a mere mortal.

Ok, but why would it be intermittent, in that case?