cmd/tsconnect: find the build dir independently of -trimpath

trimmed builds don't have absolute path information in executable
metadata, which leads the runtime.Caller approach failing
mysteriously in yarn with complaints about relative package paths.

So, instead of using embedded package metadata to find paths,
expect that we're being invoked within the tailscale repo, and
locate the tsconnect directory that way.

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/tsconnect/common.go

@@ -28,10 +28,13 @@ const (
 func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 	// Change cwd to to where this file lives -- that's where all inputs for
 	// esbuild and other build steps live.
-	if _, filename, _, ok := runtime.Caller(0); ok {
-		if err := os.Chdir(path.Dir(filename)); err != nil {
-			return nil, fmt.Errorf("Cannot change cwd: %w", err)
-		}
+	root, err := findRepoRoot()
+	if err != nil {
+		return nil, err
+	}
+	tsConnectDir := filepath.Join(root, "cmd", "tsconnect")
+	if err := os.Chdir(tsConnectDir); err != nil {
+		return nil, fmt.Errorf("Cannot change cwd: %w", err)
 	}
 	if err := installJSDeps(); err != nil {
 		return nil, fmt.Errorf("Cannot install JS deps: %w", err)
@@ -67,6 +70,22 @@ func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 	}, nil
 }
 
+func findRepoRoot() (string, error) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return "", err
+	}
+	for {
+		if _, err := os.Stat(path.Join(cwd, "go.mod")); err == nil {
+			return cwd, nil
+		}
+		if cwd == "/" {
+			return "", fmt.Errorf("Cannot find repo root")
+		}
+		cwd = path.Dir(cwd)
+	}
+}
+
 func commonPkgSetup(dev bool) (*esbuild.BuildOptions, error) {
 	buildOptions, err := commonSetup(dev)
 	if err != nil {