Fall back to develocity.server for cache server

When the cache server hasn't been explicitly configured, we should
fall back to using the value of develocity.server. Doing so will make
it easier to use a  Develocity instance other than ge.spring.io as
only develocity.server has to be configured.

Closes gh-78

src/main/java/io/spring/ge/conventions/gradle/BuildCacheConventions.java

@@ -52,11 +52,10 @@ public void execute(BuildCacheConfiguration buildCache) {
 			String cacheServer = this.env.get("DEVELOCITY_CACHE_SERVER");
 			if (cacheServer == null) {
 				cacheServer = serverOfCacheUrl(this.env.get("GRADLE_ENTERPRISE_CACHE_URL"));
-				if (cacheServer == null) {
-					cacheServer = "https://ge.spring.io";
-				}
 			}
-			remote.setServer(cacheServer);
+			if (cacheServer != null) {
+				remote.setServer(cacheServer);
+			}
 			String accessKey = this.env.get("DEVELOCITY_ACCESS_KEY");
 			if (accessKey == null) {
 				accessKey = this.env.get("GRADLE_ENTERPRISE_ACCESS_KEY");

src/test/java/io/spring/ge/conventions/gradle/BuildCacheConventionsTests.java

@@ -51,7 +51,7 @@ void localCacheIsEnabled() {
 	void remoteCacheIsEnabled() {
 		new BuildCacheConventions(DevelocityBuildCache.class).execute(this.buildCache);
 		assertThat(this.buildCache.remote.isEnabled()).isTrue();
-		assertThat(this.buildCache.remote.getServer()).isEqualTo("https://ge.spring.io");
+		assertThat(this.buildCache.remote.getServer()).isNull();
 		assertThat(this.buildCache.remote.isPush()).isFalse();
 	}