Don't modify list returned by Project#getRepositories()

The returned list is mutable and can lead to an exception like the following when directly modified:

```
org.gradle.api.InvalidUserCodeException: Build was configured to prefer settings repositories over project repositories but repository 'MavenRepo' was added by unknown code
        at org.gradle.internal.management.DefaultDependencyResolutionManagement.repoMutationDisallowedOnProject(DefaultDependencyResolutionManagement.java:199)
        at org.gradle.internal.ImmutableActionSet$SetWithFewActions.execute(ImmutableActionSet.java:285)
        at org.gradle.api.internal.DefaultDomainObjectCollection.doAdd(DefaultDomainObjectCollection.java:262)
        at org.gradle.api.internal.DefaultNamedDomainObjectCollection.doAdd(DefaultNamedDomainObjectCollection.java:114)
        at org.gradle.api.internal.DefaultDomainObjectCollection.addAll(DefaultDomainObjectCollection.java:302)
        at org.gradle.api.internal.DefaultNamedDomainObjectCollection.addAll(DefaultNamedDomainObjectCollection.java:134)
        at org.openrewrite.gradle.marker.GradleProjectBuilder.gradleProject(GradleProjectBuilder.java:62)
        at org.openrewrite.gradle.isolated.DefaultProjectParser.parseGradleFiles(DefaultProjectParser.java:890)
        at org.openrewrite.gradle.isolated.DefaultProjectParser.parse(DefaultProjectParser.java:831)
        ... 167 more
```

Fixes: #12

model/src/main/java/org/openrewrite/gradle/marker/GradleProjectBuilder.java

@@ -55,7 +55,7 @@ public static GradleProject gradleProject(Project project) {
             pluginRepositories.addAll(mapRepositories(settings.getPluginManagement().getRepositories()));
             pluginRepositories.addAll(mapRepositories(settings.getBuildscript().getRepositories()));
         }
-        List<ArtifactRepository> repositories = project.getRepositories();
+        List<ArtifactRepository> repositories = new ArrayList<>(project.getRepositories());
         if(GradleVersion.current().compareTo(GradleVersion.version("6.8")) >= 0) {
             Settings settings = ((DefaultGradle)project.getGradle()).getSettings();
             //noinspection UnstableApiUsage