Fix compilation when TBB support is disabled

[d-torrance]

A couple lines of code were on the wrong side of WITH_TBB conditionals, causing the following compilation errors when building without TBB support:

CXX f4/f4.cpp
../../../../Macaulay2/e/f4/f4.cpp: In constructor ‘F4GB::F4GB(const VectorArithmetic*, const MonomialInfo*, const FreeModule*, M2_bool, int, M2_arrayint, int, M2_bool, int, int)’:
../../../../Macaulay2/e/f4/f4.cpp:72:7: error: class ‘F4GB’ does not have any field named ‘mNumThreads’
   72 |       mNumThreads(mtbb::numThreads(numThreads)),
      |       ^~~~~~~~~~~
../../../../Macaulay2/e/f4/f4.cpp: In member function ‘void F4GB::gauss_reduce(bool)’:
../../../../Macaulay2/e/f4/f4.cpp:688:24: error: ‘spair_rows’ was not declared in this scope
  688 |   for (auto i = 0; i < spair_rows.size(); i++)
      |                        ^~~~~~~~~~

This is a draft for now with a commit to run the GitHub builds w/o TBB support, which I'll remove if/when the builds succeed.

[d-torrance]

It turns out we were skipping a whole chunk of the Gaussian reduction algorithm when TBB was disabled! I think I fixed it (at least all the previously failing EngineTests tests are passing for me locally)...

[d-torrance]

Yay the builds passed!

https://github.com/Macaulay2/M2/actions/runs/17417315752?pr=3970

I'll remove the last commit and restore building with TBB support in the GitHub builds