fix: Scheduled rollout cause a data race

[thomaspoignant]

Description

As reported in issue #2256, in some cases we can have a data race when using a scheduled rollout.

This PR fixes the issue by creating a deep copy of the flag before applying the scheduled steps.

Closes issue(s)

Resolve #2256

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have followed the contributing guide

[netlify]

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Name	Link
🔨 Latest commit	592dd1a
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/66d611378020b20008949c37

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.02%. Comparing base (5f0e47c) to head (592dd1a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/flag/internal_flag.go	95.45%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2309      +/-   ##
==========================================
- Coverage   86.03%   86.02%   -0.01%     
==========================================
  Files         102      102              
  Lines        3730     3743      +13     
==========================================
+ Hits         3209     3220      +11     
- Misses        398      399       +1     
- Partials      123      124       +1     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud