Update unix build instructions. #29864

<!-- *** Please remove the following help text before submitting: *** Pull requests without a rationale and clear improvement may be closed immediately. GUI-related pull requests should be opened against https://github.com/bitcoin-core/gui first. See CONTRIBUTING.md -->

<!-- Please provide clear motivation for your patch and explain how it improves Bitcoin Core user experience or Bitcoin Core developer experience significantly: * Any test improvements or new tests that improve coverage are always welcome. * All other changes should have accompanying unit tests (see `src/test/`) or functional tests (see `test/`). Contributors should note which tests cover modified code. If no tests exist for a region of modified code, new tests should accompany the change. * Bug fixes are most welcome when they come with steps to reproduce or an explanation of the potential issue as well as reasoning for the way the bug was fixed. * Features are welcome, but might be rejected due to design or scope issues. If a feature is based on a lot of dependencies, contributors should first consider building the system outside of Bitcoin Core, if possible. * Refactoring changes are only accepted if they are required for a feature or bug fix or otherwise improve developer experience significantly. For example, most "code style" refactoring changes require a thorough explanation why they are useful, what downsides they have and why they *significantly* improve developer experience or avoid serious programming bugs. Note that code style is often a subjective matter. Unless they are explicitly mentioned to be preferred in the [developer notes](/doc/developer-notes.md), stylistic code changes are usually rejected. -->

<!-- Bitcoin Core has a thorough review process and even the most trivial change needs to pass a lot of eyes and requires non-zero or even substantial time effort to review. There is a huge lack of active reviewers on the project, so patches often sit for a long time. -->

I ran into several rough edges when attempting to build on Debian. I wanted to contribute these changes in the hopes that they help others.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage

For detailed information about the code coverage, see the test coverage report.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process. A summary of reviews will appear here.

You will have to check out the documentation of the commit you are trying to compile.

For example, if you want to compile 27.x, you will have to use https://github.com/bitcoin/bitcoin/blob/27.x/doc/build-unix.md , not the one of master.