Add environment variable for enabling Go extraction reconcilation

This will enable us to test this behavior in PR checks.
Also simplify and add some more detailed documentation.

lib/analyze-action.js

@@ -119,13 +119,28 @@ async function run() {
         const repositoryNwo = (0, repository_1.parseRepositoryNwo)(util.getRequiredEnvParam("GITHUB_REPOSITORY"));
         const gitHubVersion = await (0, api_client_1.getGitHubVersionActionsOnly)();
         const featureFlags = new feature_flags_1.GitHubFeatureFlags(gitHubVersion, apiDetails, repositoryNwo, logger);
-        if (await featureFlags.getValue(feature_flags_1.FeatureFlag.GolangExtractionReconciliationEnabled)) {
-            // Run autobuilder for Go, unless it's already been run or user built manually
-            if (languages_1.Language.go in config.languages &&
-                process.env["CODEQL_ACTION_DID_AUTOBUILD_GOLANG"] !== "true" &&
-                !doesGoExtractionOutputExist(config)) {
-                await (0, autobuild_1.runAutobuild)(languages_1.Language.go, config, logger);
-            }
+        // When Go extraction reconciliation is enabled, either via the feature flag
+        // or an environment variable, we will attempt to autobuild Go to preserve
+        // compatibility for users who have set up Go using a legacy scanning style
+        // CodeQL workflow, i.e. one without an autobuild step or manual build
+        // steps.
+        //
+        // - We detect whether an autobuild step is present by checking the
+        // `CODEQL_ACTION_DID_AUTOBUILD_GOLANG` environment variable, which is set
+        // when the autobuilder is invoked.
+        // - We approximate whether manual build steps are present by looking at
+        // whether any extraction output already exists for Go.
+        if (
+        // Only proceed if the beta Go extraction reconciliation behavior is
+        // enabled.
+        (process.env["CODEQL_ACTION_RECONCILE_GO_EXTRACTION"] === "true" ||
+            (await featureFlags.getValue(feature_flags_1.FeatureFlag.GolangExtractionReconciliationEnabled))) &&
+            languages_1.Language.go in config.languages &&
+            // This captures whether the autobuilder has already been invoked
+            process.env["CODEQL_ACTION_DID_AUTOBUILD_GOLANG"] !== "true" &&
+            // This captures whether a user has added manual build steps for Go
+            !doesGoExtractionOutputExist(config)) {
+            await (0, autobuild_1.runAutobuild)(languages_1.Language.go, config, logger);
         }
         dbCreationTimings = await (0, analyze_1.runFinalize)(outputDir, threads, memory, config, logger, featureFlags);
         if (actionsUtil.getRequiredInput("skip-queries") !== "true") {