[MINSTALL-205] Warning when using install-file in parallel builds

**[Gili](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=cowwoc)** opened **[MINSTALL-205](https://issues.apache.org/jira/browse/MINSTALL-205?redirect=false)** and commented

ePer the online documentation, `install-file`:
* Executes as an aggregator goal.
* The goal is thread-safe and supports parallel builds.

But when I use it in a parallel build I get this warning:

`[WARNING] *****************************************************************`
`[WARNING] * The                                                           *`
`[WARNING] * org.apache.maven.plugins:maven-install-plugin:3.1.3:install-file *`
`[WARNING] * aggregator mojo is already being executed in this parallel    *`
`[WARNING] * build, those kind of mojos require exclusive access to        *`
`[WARNING] * reactor to prevent race conditions. This mojo execution will  *`
`[WARNING] * be blocked until the aggregator mojo is done.                 *`
`[WARNING] *****************************************************************`

Expected behavior: If the plugin is safe for use in parallel builds, it should not trigger this warning.

---

**Affects:** 3.1.3, 4.0.0-beta-1

Start by reproducing the warning from the install-file goal in a parallel build, using the affected versions 3.1.3 or 4.0.0-beta-1. Investigate how the goal is marked as an aggregator and thread-safe, then verify that a parallel build no longer emits the exclusive-reactor warning when the behavior is corrected.

Written by the indexing model from the issue text.