/usr/bin can be prefixed to PATH

juju-ci-tools prefixes the directory containing juju to PATH. This is so that juju finds the right copy of whatever plugins it ships with. But it can backfire when /usr/bin/juju* is used. If two copies of a plugin are installed, e.g. one in /usr/local/bin, the other in /usr/bin, this PATH manipulation will cause the /usr/bin version to be selected, instead of the /usr/local/bin version.

This happened to me, and I mistakenly filed bug #1588908 as a result.

juju-ci-tools should select the plugins packaged with juju, but no others. It should not interfere with other plugin lookup, so it should not force /usr/bin early into the PATH.