python-builder: don't force siblings install

It seems I added this --force with the original commit
(I4943ae723b06b0ad808e7c7f20788109e21aa8bf) but I'm not really sure
why.  If we have built any siblings their wheels should have higher
version numbers (e.g. like pbr versioning with a "dev" on it).

Thus we shouldn't need to force the wheels to be installed.  The
--force here causes a lot of uninstalls that take up quite a bit of
time, especialy under emulation.

Change-Id: I88b824058dc1cee90bfe4c8c4fd43a86472bc478

docker/python-builder/scripts/install-from-bindep

@@ -14,7 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-set -e
+set -ex
 
 apt-get update
 DEBIAN_FRONTEND=noninteractive apt-get -y install $(cat /output/bindep/run.txt)
@@ -43,11 +43,8 @@ if [ -f /output/packages.txt ] ; then
   # image.
   pip install $CONSTRAINTS --cache-dir=/output/wheels -r /output/packages.txt $EXTRAS
 else
-  # Install the wheels.  Use --force here because sibling wheels might
-  # be built with the same version number as the latest release, but we
-  # really want the speculatively built wheels installed over any
-  # automatic dependencies.
-  pip install $CONSTRAINTS --force --cache-dir=/output/wheels /output/wheels/*.whl $EXTRAS
+  # Install the wheels.
+  pip install $CONSTRAINTS --cache-dir=/output/wheels /output/wheels/*.whl $EXTRAS
 fi
 
 # clean up after ourselves