build: honor CMAKE_INSTALL_LIBEXECDIR in wrapper #36037

Fixes #35785

Internal binaries are installed to CMAKE_INSTALL_LIBEXECDIR. That defaults to libexec, but a distro can set it to something else (Arch uses lib). The bitcoin wrapper always looked in ../libexec under the install prefix, so those builds fail after cmake --install for bitcoin-node, chainstate, and the other internal binaries.

The wrapper now uses the same CMake value.

tool_bitcoin.py copies the wrapper into a fake prefix/bin and bitcoind only into prefix/<LIBEXECDIR>, then runs it by absolute path.

Putting the binary in a different directory should fail. Normal functional tests miss this because every binary sits next to the wrapper in the build tree.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36037.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36022 (test: add coverage for bitcoin wrapper argument handling by cyb3ralbert)
• #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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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/32378100542/job/97015882585} _{LLM reason (✨ experimental): CI failed due to a linted commit message format error: the commit subject line wasn’t followed by a blank line before the body.}

<details><summary>Hints</summary>

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• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

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