s/buildId/buildID/ as per guidelines (#90)

internal/driver/fetch.go

 	}
 	// Replace executable filename/buildID with the overrides from source.
 	// Assumes the executable is the first Mapping entry.
-	if execName, buildId := s.ExecName, s.BuildID; execName != "" || buildId != "" {
+	if execName, buildID := s.ExecName, s.BuildID; execName != "" || buildID != "" {
 		if len(p.Mapping) == 0 {
 			// If there are no mappings, add a fake mapping to attempt symbolization.
 			// This is useful for some profiles generated by the golang runtime, which
 		if execName != "" {
 			m.File = execName
 		}
-		if buildId != "" {
-			m.BuildID = buildId
+		if buildID != "" {
+			m.BuildID = buildID
 		}
 	}
 }