Fixes spurious error messages in tests

Previously, `isAnalyzingDefaultBranch` was failing because there are
some missing env vars: `GITHUB_SHA`, `GITHUB_REF`, and
`GITHUB_EVENT_PATH`. Also, `checkout_path` is missing as an input.

Rather than trying to set them to mock values, which would require
setting the paths to existing paths in the file system, I chose to stub
the entire function. I think this is fine since the point of the test
is to check the ram and threads values, not testing the
`isAnalyzingDefaultBranch` function.

lib/analyze-action-env.test.js

@@ -45,6 +45,7 @@ const util = __importStar(require("./util"));
             .stub(actionsUtil, "createStatusReportBase")
             .resolves({});
         sinon.stub(actionsUtil, "sendStatusReport").resolves(true);
+        sinon.stub(actionsUtil, "isAnalyzingDefaultBranch").resolves(true);
         const gitHubVersion = {
             type: util.GitHubVariant.DOTCOM,
         };