resource: Optimize integrity string generation

resource: Optimize integrity string generation

Remove use of fmt.Sprintf for simple string concatenation.  A simple
change for a small perf boost.

```
name         old time/op    new time/op    delta
Integrity-4     525ns ± 2%     268ns ± 2%  -48.92%  (p=0.000 n=10+10)

name         old alloc/op   new alloc/op   delta
Integrity-4      144B ± 0%      112B ± 0%  -22.22%  (p=0.000 n=10+10)

name         old allocs/op  new allocs/op  delta
Integrity-4      5.00 ± 0%      3.00 ± 0%  -40.00%  (p=0.000 n=10+10)
```