Fix correctness of offsetgen

[RonFed]

We saw a few cases where the offsetgen tool produced a wrong value.
More specifically, a valid cached offset was re-written as nil in https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/1044/files#
I couldn't reproduce this locally.

The main suspect is the following function:

func (i *Inspector) do(ctx context.Context, j job) (out []result, err error) {
	var uncachedIndices []int
	for _, f := range j.Fields {
		o, ok := i.Cache.GetOffset(j.AppVer, f)
		out = append(out, result{
			StructField: f,
			Version:     j.AppVer,
			Offset:      o.Offset,
			Valid:       o.Valid,
		})
		if !ok {
			uncachedIndices = append(uncachedIndices, len(out)-1)
		}
	}

	if len(uncachedIndices) == 0 {
		return out, nil
	}

	app, err := newApp(ctx, i.log, j)
	buildErr := &errBuild{}
	if errors.As(err, &buildErr) {
		i.log.Debug(
			"failed to build app, skipping",
			"version", j.AppVer,
			"src", j.Renderer.src,
			"Go", j.Builder.GoImage,
			"rc", buildErr.ReturnCode,
			"stdout", buildErr.Stdout,
			"stderr", buildErr.Stderr,
		)
		return out, nil
	} else if err != nil {
		return nil, err
	}
	defer app.Close()

	for _, i := range uncachedIndices {
		out[i].Offset, out[i].Valid = app.GetOffset(out[i].StructField)
	}

	return out, nil
}

We can improve on the error handling there I think,
@MrAlias WDYT?