Use buildkite branch for transport checks if not in PR CI (elastic#136647) (elastic#136719)

When testing intake and other periodic jobs, transport version build
checks should use the branch being tested, not main. This commit falls
back to the branch reported by buildkite.

Author: rjernst

build-tools-internal/src/integTest/groovy/org/elasticsearch/gradle/internal/transport/TransportVersionValidationFuncTest.groovy

@@ -312,8 +312,7 @@ class TransportVersionValidationFuncTest extends AbstractTransportVersionFuncTes
         result.task(":myserver:validateTransportVersionResources").outcome == TaskOutcome.SUCCESS
     }
 
-    def "modification checks use PR base branch in CI"() {
-        given:
+    void baseBranchSetup() {
         // create 9.1 branch
         execute("git checkout -b 9.1")
         // setup 9.1 upper bound as if 9.2 was just branched, so no patches yet
@@ -342,10 +341,27 @@ class TransportVersionValidationFuncTest extends AbstractTransportVersionFuncTes
                 currentUpperBoundName = '9.1'
             }
         """
+    }
+
+    def "modification checks use PR base branch in CI"() {
+        given:
+        baseBranchSetup()
+
+        when:
+        def result = gradleRunner("validateTransportVersionResources")
+            .withEnvironment(Map.of("BUILDKITE_PULL_REQUEST_BASE_BRANCH", "9.1", "BUILDKITE_BRANCH", "foobar")).build()
+
+        then:
+        result.task(":myserver:validateTransportVersionResources").outcome == TaskOutcome.SUCCESS
+    }
+
+    def "modification checks use buildkite base branch in CI if not in PR"() {
+        given:
+        baseBranchSetup()
 
         when:
         def result = gradleRunner("validateTransportVersionResources")
-            .withEnvironment(Map.of("BUILDKITE_PULL_REQUEST_BASE_BRANCH", "9.1")).build()
+            .withEnvironment(Map.of("BUILDKITE_BRANCH", "9.1")).build()
 
         then:
         result.task(":myserver:validateTransportVersionResources").outcome == TaskOutcome.SUCCESS

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/TransportVersionResourcesService.java

@@ -287,7 +287,12 @@ private String findUpstreamRef() {
         // default the branch name to look at to that which a PR in CI is targeting
         String branchName = System.getenv("BUILDKITE_PULL_REQUEST_BASE_BRANCH");
         if (branchName == null || branchName.strip().isEmpty()) {
-            branchName = "main";
+            // fallback to the local branch being tested in CI
+            branchName = System.getenv("BUILDKITE_BRANCH");
+            if (branchName == null || branchName.strip().isEmpty()) {
+                // fallback to main if we aren't in CI
+                branchName = "main";
+            }
         }
         List<String> remoteNames = List.of(remotesOutput.split("\n"));
         if (remoteNames.contains(UPSTREAM_REMOTE_NAME) == false) {