fix: respect onnxruntime_USE_PREINSTALLED_EIGEN option

[cloudhan]

Description

In #21348, eigen find logic is changed if the eigen package is present on the system, causing

FAILED: CMakeFiles/onnxruntime_common.dir/home/username/onnxruntime/onnxruntime/core/platform/posix/env.cc.o 
ccache /usr/bin/c++ -DCPUINFO_SUPPORTED_PLATFORM=1 -DEIGEN_MPL2_ONLY -DEIGEN_USE_THREADS -DNSYNC_ATOMIC_CPP11 -DONLY_C_LOCALE=0 -DORT_ENABLE_STREAM -DPLATFORM_POSIX -DUSE_QNN=1 -D_GNU_SOURCE -I/home/username/onnxruntime/build_qnn/RelWithDebInfo/_deps/utf8_range-src -I/home/username/onnxruntime/include/onnxruntime -I/home/username/onnxruntime/include/onnxruntime/core/session -I/home/username/onnxruntime/build_qnn/RelWithDebInfo/_deps/pytorch_cpuinfo-src/include -I/home/username/onnxruntime/build_qnn/RelWithDebInfo/_deps/google_nsync-src/public -I/home/username/onnxruntime/build_qnn/RelWithDebInfo -I/home/username/onnxruntime/onnxruntime -I/home/username/onnxruntime/build_qnn/RelWithDebInfo/_deps/abseil_cpp-src -I/home/username/onnxruntime/build_qnn/RelWithDebInfo/_deps/date-src/include -I/usr/include/eigen3 -I/home/username/onnxruntime/build_qnn/RelWithDebInfo/_deps/safeint-src -isystem /home/username/onnxruntime/build_qnn/RelWithDebInfo/_deps/gsl-src/include -ffunction-sections -fdata-sections -Wno-restrict  -DCPUINFO_SUPPORTED -O2 -g -DNDEBUG -std=gnu++17 -fPIC -Wall -Wextra -Wno-deprecated-copy -Wno-nonnull-compare -Werror -MD -MT CMakeFiles/onnxruntime_common.dir/home/username/onnxruntime/onnxruntime/core/platform/posix/env.cc.o -MF CMakeFiles/onnxruntime_common.dir/home/username/onnxruntime/onnxruntime/core/platform/posix/env.cc.o.d -o CMakeFiles/onnxruntime_common.dir/home/username/onnxruntime/onnxruntime/core/platform/posix/env.cc.o -c /home/username/onnxruntime/onnxruntime/core/platform/posix/env.cc
In file included from /home/username/onnxruntime/onnxruntime/core/platform/posix/env.cc:57:
/home/username/onnxruntime/include/onnxruntime/core/platform/EigenNonBlockingThreadPool.h: In constructor ‘onnxruntime::concurrency::ThreadPoolTempl<Environment>::ThreadPoolTempl(const CHAR_TYPE*, int, bool, Environment&, const onnxruntime::ThreadOptions&)’:
/home/username/onnxruntime/include/onnxruntime/core/platform/EigenNonBlockingThreadPool.h:789:21: error: ‘class Eigen::MaxSizeVector<Eigen::MaxSizeVector<unsigned int> >’ has no member named ‘emplace_back’
  789 |       all_coprimes_.emplace_back(i);
      |                     ^~~~~~~~~~~~

Because if find the very old version of eigen which do not have the emplace_back defined.

[snnn]

When the option is set, user should also set "eigen_SOURCE_PATH" to point to a preinstalled eigen path. This is how the option was used in the past.

[snnn]

onnxruntime_USE_PREINSTALLED_EIGEN is not used for indicating whether find_package should be used for looking for Eigen or not. It is a deprecated feature. I even couldn't remember why it was added. It is used with another cmake variable.

By default, cmake always call find_package first, unless FETCHCONTENT_TRY_FIND_PACKAGE_MODE is set to NEVER.