Fix Maven plugin caching again

[arrdem]

As of at least build 477, a whole bunch of time is spent downloading Maven plugins whose versions have changed. Because Maven automatically updates. Which is great. Getting pretty sick of this crap.

Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-utils/3.0/plexus-utils-3.0.pom
...
Downloading: https://repo.maven.apache.org/maven2/org/apache/maven/maven-archiver/2.4.1/maven-archiver-2.4.1.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-archiver/1.0/plexus-archiver-1.0.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-components/1.1.17/plexus-components-1.1.17.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus/2.0.5/plexus-2.0.5.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-io/1.0/plexus-io-1.0.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-io/1.0/plexus-io-1.0.jar
...
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-utils/3.0.5/plexus-utils-3.0.5.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus/3.1/plexus-3.1.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-digest/1.0/plexus-digest-1.0.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-components/1.1.7/plexus-components-1.1.7.pom
Downloading: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-container-default/1.0-alpha-8/plexus-container-default-1.0-alpha-8.pom

[arrdem]

See prior "fun" on this one, #30