Fix local activity retry backoff

[yiminc]

What was changed

Fix local activity retry backoff calculation.

Why?

Attempt used to start from 0, then it was changed to start from 1, but backoff calculation wasn't updated accordingly.

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

[vitarb]

Is this relevant for local activity only, or should we change general retry logic here

sdk-go/internal/common/retry/interceptor.go

Line 136 in 00ce39a

next := float64(rc.initialInterval) * math.Pow(rc.backoffCoefficient, float64(attempt))

as well?

[yiminc]

@vitarb , the backoff changed in this PR is only used by local activity and the unit test framework to calculate activity/workflow retry backoff in unit test. I'm not sure about the grpc_retry as I don't know what is the attempt used there. It is worth double check.