Fix automatic toolchain downloads again.

Automatic toolchains downloads aren't necessary for our CI, which pre-downloads the toolchains that it needs. But automatic toolchains downloads remain necessary for local users who don't already have the necessary JDKs downloaded and registered.

I had the configuration right initially, and then I broke it in cl/711746683 without noticing because (as a comment in `pom.xml` observes) you don't notice unless you clear out your local Maven configuration before running. (I still haven't done that now, as of this writing, because I'm in the process of using one of the Maven-configured JDKs for something else....)

This fix also addresses this warning:

```
[WARNING] Some problems were encountered while building the effective model for com.google.guava:guava:bundle:HEAD-jre-SNAPSHOT
[WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.mvnsearch:toolchains-maven-plugin @ com.google.guava:guava-parent:HEAD-jre-SNAPSHOT, /usr/local/google/home/cpovirk/clients/guava-black/guava/pom.xml, line 389, column 17
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.google.guava:guava-parent:pom:HEAD-jre-SNAPSHOT
[WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.mvnsearch:toolchains-maven-plugin @ line 389, column 17
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
```

RELNOTES=n/a
PiperOrigin-RevId: 715914892

android/pom.xml

@@ -192,6 +192,7 @@
         <plugin>
           <groupId>org.mvnsearch</groupId>
           <artifactId>toolchains-maven-plugin</artifactId>
+          <version>4.5.0</version>
           <executions>
             <!--
             We can apparently have only one <jdk> per execution: Others are silently ignored :(
@@ -391,11 +392,6 @@
           <artifactId>build-helper-maven-plugin</artifactId>
           <version>3.4.0</version>
         </plugin>
-        <plugin>
-          <groupId>org.mvnsearch</groupId>
-          <artifactId>toolchains-maven-plugin</artifactId>
-          <version>4.5.0</version>
-        </plugin>
       </plugins>
     </pluginManagement>
   </build>

pom.xml

@@ -193,6 +193,7 @@
         <plugin>
           <groupId>org.mvnsearch</groupId>
           <artifactId>toolchains-maven-plugin</artifactId>
+          <version>4.5.0</version>
           <executions>
             <!--
             We can apparently have only one <jdk> per execution: Others are silently ignored :(
@@ -386,11 +387,6 @@
           <artifactId>build-helper-maven-plugin</artifactId>
           <version>3.4.0</version>
         </plugin>
-        <plugin>
-          <groupId>org.mvnsearch</groupId>
-          <artifactId>toolchains-maven-plugin</artifactId>
-          <version>4.5.0</version>
-        </plugin>
       </plugins>
     </pluginManagement>
   </build>