meson: Fix rpath clearing

Meson allows projects to set `build_rpath` property, containing paths
that will be added during build but will be removed when installing.

When Meson removes build_rpath from `DT_RUNPATH` entry, it just writes
the shorter ␀-terminated new rpath over the old one to reduce
the risk of potentially breaking the ELF files
(when the linker does string de-duplication or something).
But this can cause much bigger problem for Nix, as it can produce
cut-in-half-by-␀ store path references.

For example, in systemd’s libudev, it was removing three `$ORIGIN`-relative paths from

    $ORIGIN/../libsystemd:$ORIGIN/../basic:$ORIGIN/../shared:…␀

resulting in the following `DT_RUNPATH` entry:

    …␀store/v589pqjhvxrj73g3r0xb41yr84z5pwb7-gcc-9.3.0-lib/lib␀

We previously handled this in `fix-rpath.patch` but the method we prevent
Meson from removing paths added to rpath through `NIX_LDFLAGS` was changed
during 0.55.0 update and I forgot about this second purpose of the patch.

Let’s re-add this clearing code, as it worked without issues for a long time.

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