Always install latest pbr

When not installing pbr from source always install the latest version of
pbr. It turns out that python-pbr is a system package that satisfies
many of our requirements files pbr requirements but breaks under
setuptools 8.0. Fix this by passing the -U flag to pip when installing
pbr so that we install the latest version of pbr always.

Note that we likely need to make this more generic to avoid other system
package leakage when installing packages not from source.

We should also probably bump our pbr requirements across the board to
reflect the new setuptools 8.0 world needs.

Change-Id: I23dd21cea37d26f879aa8d864ee7d371e70221ea
Fixes-bug: 1405318