fix(mining): Wrong timestamp sent to _make_block_template

[msbrogli]

Motivation

PR #822 was not enough to fix the flaky tests because the caller of _make_block_template() was sending the wrong current_timestamp.

Acceptance Criteria

1. Fix the calculation of current_timestamp in HathorManager.make_block_template().

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9cee806) 84.55% compared to head (25b3983) 84.51%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
- Coverage   84.55%   84.51%   -0.05%     
==========================================
  Files         269      269              
  Lines       22234    22234              
  Branches     3397     3397              
==========================================
- Hits        18800    18791       -9     
- Misses       2765     2770       +5     
- Partials      669      673       +4     

Files	Coverage Δ
hathor/manager.py	80.73% <100.00%> (ø)

... and 3 files with indirect coverage changes

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