cmake: Set top-level target output locations

[hebasto]

While testing #1551, I noticed that when cross-compiling a shared library with examples for Windows, the ctest fails to run examples with Wine. Adjusting the PATH variable in

secp256k1/examples/CMakeLists.txt

Lines 16 to 18 in 4af241b

	set_tests_properties(${test_name} PROPERTIES
	ENVIRONMENT "PATH=$<TARGET_FILE_DIR:secp256k1>;$ENV{PATH}"
	)

does not help because WINEPATH is expected.

Another issue with the current implementation is that the examples cannot run individually on Windows.

This PR resolves both issues by reverting the implementation from #1290 in favour of the reworked and improved implementation from #1233.

[real-or-random]

ACK 3779472

It's probably a good idea that the binaries just work without the need to set a PATH. And it's okay to collect all binary artifacts in a single directory, we don't have that many.

[real-or-random]

What are your commands for building on wine?

[hebasto]

What are your commands for building on wine?

On Ubuntu 24.04 with the wine-binfmt package installed:

$ cmake -B build --toolchain cmake/x86_64-w64-mingw32.toolchain.cmake -DSECP256K1_BUILD_EXAMPLES=ON
$ cmake --build build
$ ctest --test-dir build
Internal ctest changing into directory: /home/hebasto/git/secp256k1/secp256k1/build
Test project /home/hebasto/git/secp256k1/secp256k1/build
    Start 1: noverify_tests
1/6 Test #1: noverify_tests ...................   Passed   52.72 sec
    Start 2: tests
2/6 Test #2: tests ............................   Passed  111.58 sec
    Start 3: exhaustive_tests
3/6 Test #3: exhaustive_tests .................   Passed    8.01 sec
    Start 4: ecdsa_example
4/6 Test #4: ecdsa_example ....................   Passed    1.35 sec
    Start 5: ecdh_example
5/6 Test #5: ecdh_example .....................   Passed    1.34 sec
    Start 6: schnorr_example
6/6 Test #6: schnorr_example ..................   Passed    1.32 sec

100% tests passed, 0 tests failed out of 6

Total Test time (real) = 176.31 sec
$ ./build/bin/schnorr_example.exe 
Is the signature valid? true
Secret Key: 0x964362f6515fbb691dee49b936c8ba46dadd505eb4aa27f5db9b1ccf510f04a8
Public Key: 0x286123fee26ecfc75ec8bd6d4e175a9b4fa729e7d2bcd4034661a33dee600d80
Signature: 0xef0aa23ae13fd06710e31204a7a8ab52f0b726159b3cb2344c090220ed32d6dda958c9f7b3383c3fa7524a7f7ed54bf381c8a3bab1f8561072887dae6bf7bfe1

[fanquake]

implementation from #1233.

cc @theuni