Set SOURCE_DATE_EPOCH

[martincostello]

Set the SOURCE_DATE_EPOCH to the date of the checked-out Git commit to use for NuGet's DeterministicTimestamp in .NET 11.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (0dfd360) to head (ec2ffde).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3997   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files         111      111           
  Lines        3958     3958           
  Branches      801      801           
=======================================
  Hits         3762     3762           
  Misses        196      196           

Flag	Coverage Δ
Linux	95.04% <ø> (ø)
Windows	95.04% <ø> (ø)
macOS	95.04% <ø> (ø)

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