Add clang to CI, simplify matrix

[aaron-skydio]

Adds clang 7 and clang 14 to CI. Also trims down the matrix a bit to only test gcc 5 with one configuration (bionic, py3.8), which keeps the full test suite under 20 jobs (the max we can run concurrently for free on public repos).

clang 6 fails with this error: https://github.com/aaron-skydio/symforce/actions/runs/3661889498/jobs/6190537015

[ 80%] Building CXX object symforce/pybind/CMakeFiles/cc_sym.dir/cc_factor.cc.o
/home/runner/work/symforce/symforce/symforce/pybind/cc_factor.cc:137:12: error: no matching function for call to 'init'
      .def(py::init(&MakeHessianFactor<sym::Key>), py::arg("hessian_func"), py::arg("keys"),
           ^~~~~~~~
/home/runner/work/symforce/symforce/build/_deps/pybind11-src/include/pybind11/pybind11.h:1912:5: note: candidate function [with Func = sym::Factor<double> (*)(std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool), Ret = pybind11::detail::initimpl::factory<sym::Factor<double> (*)(std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool), pybind11::detail::void_type (*)(), sym::Factor<double> (std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool), pybind11::detail::void_type ()>] not viable: no overload of 'MakeHessianFactor' matching 'sym::Factor<double> (*)(std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool)' for 1st argument
Ret init(Func &&f) {
    ^
/home/runner/work/symforce/symforce/build/_deps/pybind11-src/include/pybind11/pybind11.h:1900:40: note: candidate function template not viable: requires 0 arguments, but 1 was provided
detail::initimpl::constructor<Args...> init() {
                                       ^
/home/runner/work/symforce/symforce/build/_deps/pybind11-src/include/pybind11/pybind11.h:1920:5: note: candidate function template not viable: requires 2 arguments, but 1 was provided
Ret init(CFunc &&c, AFunc &&a) {
    ^
/home/runner/work/symforce/symforce/symforce/pybind/cc_factor.cc:148:12: error: no matching function for call to 'init'
      .def(py::init(&MakeHessianFactor<sym::Key, sym::Key>), py::arg("hessian_func"),
           ^~~~~~~~
/home/runner/work/symforce/symforce/build/_deps/pybind11-src/include/pybind11/pybind11.h:1912:5: note: candidate function [with Func = sym::Factor<double> (*)(std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool), Ret = pybind11::detail::initimpl::factory<sym::Factor<double> (*)(std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool), pybind11::detail::void_type (*)(), sym::Factor<double> (std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool), pybind11::detail::void_type ()>] not viable: no overload of 'MakeHessianFactor' matching 'sym::Factor<double> (*)(std::function<pybind11::tuple (const sym::Values<double> &, const std::vector<sym::index_entry_t, std::allocator<sym::index_entry_t> > &)>, const std::vector<sym::Key, std::allocator<sym::Key> > &, const std::vector<sym::Key, std::allocator<sym::Key> > &, bool)' for 1st argument
Ret init(Func &&f) {
    ^
/home/runner/work/symforce/symforce/build/_deps/pybind11-src/include/pybind11/pybind11.h:1900:40: note: candidate function template not viable: requires 0 arguments, but 1 was provided
detail::initimpl::constructor<Args...> init() {
                                       ^
/home/runner/work/symforce/symforce/build/_deps/pybind11-src/include/pybind11/pybind11.h:1920:5: note: candidate function template not viable: requires 2 arguments, but 1 was provided
Ret init(CFunc &&c, AFunc &&a) {
    ^