Minor race prevention: do not mutate callers' retry policy

[Groxx]

Detailed Description

Concurrently calling workflow.WithActivityOptions with a shared retry policy instance that does not set a backoff coefficient can trigger a race because this function mutated it unnecessarily.

The fix is easy: just don't do that :)

Impact Analysis
Fixes tests and prod that share activity-option-vars, which is somewhat common in some codebases. I'm not sure if this can lead to any actual misbehavior since it's a single-word value and it's only ever adjusted to one other value, but I'm fully on board with "never allow races no matter what".

Testing Plan
Test suite should be plenty.

Rollout Plan
Nothing special.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.59%. Comparing base (2bceb13) to head (6f53817).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/workflow.go	0.00%	2 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
internal/workflow.go	76.24% <0.00%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2bceb13...6f53817. Read the comment docs.