*/build_deb: pass env vars with quotes

*/build_deb: pass env vars with quotes

should double quote env var in command substitution, otherwise shell will
perform field splitting with the substitution, and only the first element in
it will be consumed. that's why CEPH_EXTRA_CMAKE_ARGS will always be
`-DALLOCATOR=libc` when it reaches debian/rules, even if
'CEPH_EXTRA_CMAKE_ARGS' is "-DALLOCATOR=libc -DWITH_STATIC_LIBSTDCXX=ON",
and it is passed to pbuilder like:

sudo CEPH_EXTRA_CMAKE_ARGS=$CEPH_EXTRA_CMAKE_ARGS pbuilder ...

Fixes: http://tracker.ceph.com/issues/22465
Signed-off-by: Kefu Chai <kchai@redhat.com>