fix(rpm): support rpmbuild 4.20+ per-package build directory (#1084)

RPM 4.20 introduced a per-package build directory and redefined
`%{_builddir}` accordingly (rpm-software-management/rpm#2885).

As a result, the paths generated by `rules_pkg` for `%install`, which
were relative to the previous working directory, no longer resolve:
```text
cp: cannot stat 'tests/testdata/a.ar': No such file or directory
```

This change makes the generated paths independent of `rpmbuild`'s
working directory:
- `%install` copies inputs from `%{_topdir}/BUILD`, where `make_rpm.py`
stages them,
- `%files -f` receives an absolute path,
- the same applies to regular files and tree artifact inputs.

This also removes the debuginfo-specific `../` path variant introduced
in #1006, since the same paths now work across RPM versions.

The RPM test spec relied on the working directory as well and gets the
same anchor.

RPM 4.20 also makes `buildsubdir .` usable again, while the `BUILD_SUB`
path used for RPM 4.18 and 4.19 is no longer reachable from the new
per-package build directory.

Debuginfo autodetection therefore selects:
- "centos" for RPM versions before 4.18 and from 4.20 onward,
- "fedora" for RPM 4.18 and 4.19.

Validated with `//tests/rpm/...`, `//toolchains/...`, and
`//pkg/legacy/tests/rpm/...` against RPM 4.18.2 and 6.0.1, CI covering
RPM 4.17.

Fixes #938.