Write generated source files to the build tree, not the source tree. (#14455)

My org, as part of its reproducible-build hygiene, builds projects with the source trees in read-only mode. I approached the protobuf build in the same way, but encountered this error (among others):
```
[ 30%] Built target statusor
[ 34%] Built target libprotobuf-lite
[ 45%] Built target libprotobuf
Consolidate compiler generated dependencies of target libprotoc
[ 62%] Built target libprotoc
Consolidate compiler generated dependencies of target protoc
[ 62%] Built target protoc
Consolidate compiler generated dependencies of target scoped_mock_log
[ 62%] Built target scoped_mock_log
[ 62%] Generating /home/src/protobuf/src/google/protobuf/util/message_differencer_unittest_proto3.pb.h, /home/src/protobuf/src/google/protobuf/util/message_differencer_unittest_proto3.pb.cc
/home/src/protobuf/src/google/protobuf/util/message_differencer_unittest_proto3.pb.cc: Read-only file system
make[2]: *** [CMakeFiles/libtest_common.dir/build.make:387: /home/src/protobuf/src/google/protobuf/util/message_differencer_unittest_proto3.pb.h] Error 1
make[1]: *** [CMakeFiles/Makefile2:1194: CMakeFiles/libtest_common.dir/all] Error 2
make: *** [Makefile:146: all] Error 2
```
(`/home/src` is a read-only mount)

Generated source files are effectively build artifacts, and should be written to the build tree, not the source tree. This PR accordingly enables a successful build and test run (lite, full, and conformance) with read-only sources.

(Beyond this, I would add that CMake usually does not need source file paths to be qualified with `CMAKE_SOURCE_DIR` or `CMAKE_BINARY_DIR`; in most cases it knows to look in both locations, favoring the latter if a file is in both. The CMake code could be simplified by relying on this behavior.)

Closes #14455

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/14455 from iskunk:feature/ro-source da7510c24a3c77ab09306647ef864a886b690587
PiperOrigin-RevId: 607031010