Allow self-sufficient desktop build for Github repository.

Test: OUT_DIR=$HOME/compose/androidx-main/out ALLOW_PUBLIC_REPOS=true \
      COMPOSE_DESKTOP_GITHUB_BUILD=true ANDROIDX_PROJECTS=compose \
      gradle -Pandroidx.compose.multiplatformEnabled=true \
        :compose:desktop:desktop:desktop-samples:run

Change-Id: Ic3bc3ccf802bd507b90ddb696aab2084653b051c
diff --git a/buildSrc/build.gradle b/buildSrc/build.gradle
index 71fdf91..55ed0c2 100644
--- a/buildSrc/build.gradle
+++ b/buildSrc/build.gradle
@@ -2,11 +2,8 @@
 
 buildscript {
     project.ext.supportRootFolder = project.projectDir.getParentFile()
-    repositories {
-        maven {
-            url("${supportRootFolder}/../../prebuilts/androidx/external")
-        }
-    }
+    apply from: "repos.gradle"
+    repos.addMavenRepositories(repositories)
 
     dependencies {
         classpath(libs.kotlinGradlePlugin)