build: respect CMAKE_INSTALL_LIBEXECDIR in bitcoin launcher #35789

This updates the bitcoin launcher to use the configured CMAKE_INSTALL_LIBEXECDIR when resolving internal executables from an installed bin/ layout.

The issue report showed distro builds that install internal binaries into lib/ instead of libexec/, which currently breaks commands like bitcoin -m node and bitcoin chainstate after installation. The wrapper already receives the configured install destination in CMake, so this threads the same value into bitcoin-build-config.h and uses it in the installed-wrapper lookup path.

The functional test coverage adds an installed-layout regression to tool_bitcoin.py by copying bitcoin into a temporary bin/ directory, placing test_bitcoin only in the configured internal install directory, and verifying that invoking the wrapper by explicit path succeeds. That keeps the test from falling back to $PATH while exercising the same installed-layout logic as the bug report.

Tested:

• cmake -S . -B build-wsl-libdir -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_INSTALL_LIBEXECDIR=lib -DBUILD_GUI=OFF -DENABLE_WALLET=OFF -DBUILD_BENCH=OFF -DBUILD_TX=OFF -DBUILD_UTIL=OFF -DBUILD_UTIL_CHAINSTATE=OFF -DBUILD_CLI=OFF -DBUILD_TESTS=ON -DBUILD_DAEMON=ON -DENABLE_IPC=OFF
• cmake --build build-wsl-libdir --target bitcoin bitcoind test_bitcoin -j4
• python3 build-wsl-libdir/test/functional/tool_bitcoin.py --configfile build-wsl-libdir/test/config.ini

refs #35785

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35789.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #32387 (ipc: add windows support by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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@Bortlesboat please don't submit pull requests whose description is clearly written by AI. The test instructions are not just redundant, a reviewer should know how to run the functional tests anyway, but they also don't exercise the new behaviour you have introduced here. Closing this again.