build,json: fixup fixup of arch_packages

build,json: fixup fixup of arch_packages

The commit "1bf2b3fe90 build,json: fixup missing arch_packages" fixes
the missing package architecture locally but runs $(TOPDIR)/Makefile
rather than a target specific one. While this works on local builds just
fine, it causes the buildbots to add garbage to the `arch_packages`
variable:

    cd \"/builder/shared-workdir/build\"; git log --format=%h -1
    toolchain > /builder/shared-workdir/build/tmp/.ver_check\ncmp -s
    /builder/shared-workdir/build/tmp/.ver_check
    /builder/shared-workdir/build/staging_dir/toolchain-x86_64_gcc-8.4.0_musl/stamp/.ver_check
    || { \\\n\trm -rf
    /builder/shared-workdir/build/build_dir/target-x86_64_musl
    /builder/shared-workdir/build/staging_dir/target-x86_64_musl
    /builder/shared-workdir/build/staging_dir/toolchain-x86_64_gcc-8.4.0_musl
    /builder/shared-workdir/build/build_dir/toolchain-x86_64_gcc-8.4.0_musl;
    \\\n\tmkdir -p
    /builder/shared-workdir/build/staging_dir/toolchain-x86_64_gcc-8.4.0_musl/stamp;
    \\\n\tmv /builder/shared-workdir/build/tmp/.ver_check
    /builder/shared-workdir/build/staging_dir/toolchain-x86_64_gcc-8.4.0_musl/stamp/.ver_check;
    \\\n}\nx86_64

Only the last line contains the desired string.

Future investigation should check why the build system prints this to
stdout rather than stderr.

Signed-off-by: Paul Spooren <mail@aparcar.org>