tools: fix install-* meson targets.
We should not rely on the current working directory, as it looks like meson sets it to be the source directory (even when actually in the build dir) when running custom target (so it fails). Instead meson explicitly sets MESON_SOURCE_ROOT and MESON_BUILD_ROOT environment variables for us. We should use these. This should work in every cases (whether calling from the source or build dir). Also removing the special-casing for a _build/ directory. This updated version is generic and won't assume that you used some generic naming or direct subdir under the source (even if many people might use this path and name; I, for one, certainly don't!). See: https://mesonbuild.com/Run-targets.html
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