[pzstd]: Fix `DESTDIR` handling to allow setting `BINDIR`

[pzstd]: Fix `DESTDIR` handling to allow setting `BINDIR`

Currently, setting `BINDIR` and `DESTDIR` separately is not possible, so
the command below fails, as BINDIR is set explicitly:

    $ make -j80 install PREFIX=/usr EPREFIX=/usr BINDIR=/usr/bin SBINDIR=/usr/sbin LIBEXECDIR=/usr/libexec SYSCONFDIR=/etc SHAREDSTATEDIR=/var LOCALST ATEDIR=/var LIBDIR=/usr/lib INCLUDEDIR=/usr/include DATAROOTDIR=/usr/share DATADIR=/usr/share INFODIR=/usr/share/info LOCALEDIR=/usr/share/locale MAND IR=/usr/share/man DOCDIR=/usr/share/doc/zstd DESTDIR=/dev/shm/bee-pmenzel/zstd/zstd-1.5.5-0/image -C contrib/pzstd DESTDIR=/dev/shm/bee-pmenzel/zstd/zstd-1.5.5-0/image
    make: Entering directory
    '/dev/shm/bee-pmenzel/zstd/zstd-1.5.5-0/source/contrib/pzstd' CFLAGS="  -I../../lib -I../../lib/common -I../../programs -I. -DNDEBUG -O3 -Wall -Wextra -Wno-deprecated-declarations   " LDFLAGS=" -O3 -Wall -Wextra -pedantic  " make -C ../../lib libzstd.a
    make[1]: Entering directory '/dev/shm/bee-pmenzel/zstd/zstd-1.5.5-0/source/lib'
    make[1]: Leaving directory '/dev/shm/bee-pmenzel/zstd/zstd-1.5.5-0/source/lib'
    g++ main.o ../../programs/util.o Options.o Pzstd.o SkippableFrame.o ../../lib/libzstd.a           -O3 -Wall -Wextra -pedantic    -pthread -o pzstd
    install -d -m 755 /usr/bin/
    install -m 755 pzstd /usr/bin/pzstd
    install: cannot create regular file '/usr/bin/pzstd': Permission denied
    make: *** [Makefile:116: install] Error 1

So, do not prefix `BINDIR` with `DESTDIR`, and adapt all paths for
installation. This is more common, and, for example, `programs/Makefile`
does the same.

Fixes: 8b4e84249b ("[pzstd] Fix Makefile")