cmd/build: Don't make builddeps dirs if no B-D

Removal of builddeps dirs only happens if there are no build
dependencies.  The same must be true of creation of builddeps dirs.

Fixes issues like:

    pj@debian-x60t:~/proteanos/pkg/expat$ cat control
    Maintainer: "J. Random Hacker" <jrandom@example.com>
    # Build-Depends: opkhelper-3.0
    Homepage: http://expat.sourceforge.net/
    pj@debian-x60t:~/proteanos/pkg/expat$ sudo prokit build $root -b -c -T nop .
    opkbuild: Detecting architecture and platform...
    opkbuild: Making work area...
    opkbuild: Parsing package metadata...
    opkbuild: Build system:
    opkbuild:   Architecture:  i686-linux-glibc
    opkbuild:   Platform:      dev
    opkbuild: Host system:
    opkbuild:   Architecture:  i686-linux-glibc
    opkbuild:   Platform:      dev
    ob-checkbuilddeps: Checking build dependencies...
    ob-buildenv: Setting up build environment...
    ob-unpacksource: Extracting upstream source archive...
    ob-applypatches: No patches to be applied
    ob-installplatconf: No config files to be copied
    mv: cannot move `../root/prokit/build.1556949533/builddeps' to `./../builddeps': Directory not empty
    rmdir: failed to remove `../root/prokit/build.1556949533': Directory not empty

0 files changed, 0 insertions, 0 deletions