Stop downloading existing Bazel binaries (bazelbuild#438)

Due to a bug Bazelisk never read any binaries from the cache, thus resulting in severe runtime penalties.

core/core.go

@@ -106,7 +106,7 @@ func RunBazeliskWithArgsFunc(argsFunc ArgsFunc, repos *Repositories) (int, error
 		}
 
 		baseDirectory := filepath.Join(bazeliskHome, "downloads", bazelForkOrURL)
-		bazelPath, err = downloadBazel(resolvedBazelVersion, baseDirectory, repos, downloader)
+		bazelPath, err = downloadBazelIfNecessary(resolvedBazelVersion, baseDirectory, repos, downloader)
 		if err != nil {
 			return -1, fmt.Errorf("could not download Bazel: %v", err)
 		}
@@ -404,20 +404,25 @@ func parseBazelForkAndVersion(bazelForkAndVersion string) (string, string, error
 	return bazelFork, bazelVersion, nil
 }
 
-func downloadBazel(version string, baseDirectory string, repos *Repositories, downloader DownloadFunc) (string, error) {
+func downloadBazelIfNecessary(version string, baseDirectory string, repos *Repositories, downloader DownloadFunc) (string, error) {
 	pathSegment, err := platforms.DetermineBazelFilename(version, false)
 	if err != nil {
 		return "", fmt.Errorf("could not determine path segment to use for Bazel binary: %v", err)
 	}
 
+	destDir := filepath.Join(baseDirectory, pathSegment, "bin")
 	destFile := "bazel" + platforms.DetermineExecutableFilenameSuffix()
-	destinationDir := filepath.Join(baseDirectory, pathSegment, "bin")
+
+	destPath := filepath.Join(destDir, destFile)
+	if _, err := os.Stat(destPath); err == nil {
+		return destPath, nil
+	}
 
 	if url := GetEnvOrConfig(BaseURLEnv); url != "" {
-		return repos.DownloadFromBaseURL(url, version, destinationDir, destFile)
+		return repos.DownloadFromBaseURL(url, version, destDir, destFile)
 	}
 
-	return downloader(destinationDir, destFile)
+	return downloader(destDir, destFile)
 }
 
 func copyFile(src, dst string, perm os.FileMode) error {