Resolve buildSrc's dependencies from Maven Central first

buildSrc listed only the Gradle Plugin Portal, which serves JReleaser's
plugin itself but only proxies the rest from Maven Central. On 2026-09-22
that proxy answered 404 for jreleaser-engine 1.26.0 and failed the
linux-x86-64 native job of PR #30, while the same commit built on the other
four runners. With Maven Central listed first, as in the root buildscript,
those modules come from Central directly.

Author: abashev

buildSrc/build.gradle

@@ -2,7 +2,11 @@ plugins {
     id 'groovy-gradle-plugin'
 }
 
+// JReleaser's plugin marker and jreleaser-gradle-plugin live on the Gradle Plugin Portal only; everything they
+// depend on is on Maven Central, which the Portal merely proxies. That proxy once answered 404 for
+// jreleaser-engine and failed a CI build, so Central comes first and is asked directly, as in the root buildscript.
 repositories {
+    mavenCentral()
     gradlePluginPortal()
 }