ci: Fix $BASE_ROOT_DIR installation #35879

In the ci system, BASE_ROOT_DIR has a default value that can be changed. This has problems:

• The docs do not mention that changing the value requires re-building the image, as the value is embedded.
• Many places hard-code the default value, which is confusing and brittle.

Fix all issues by adding docs and replacing the hard-coded default values with $BASE_ROOT_DIR.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35879.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35877 (build: ci/doc win64-cross build via nix by maflcko)
• #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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