fix: chain spec not deterministic

[wischli]

Description

As part of my #1756 client checks, I noticed that our ephemeral chain specs did not lead to a deterministic chain spec which blocks all our scripts to register such parachain. The root of this was introduced by myself in #1740 by adding a unix timestamp for the block rewards. Since this timestamp is completely irrelevant for ephemeral chains, I changed it to the default value. This will mint a larger amount of treasury inflation in the first session and then go back to normal.

Checklist:

• I have added Rust doc comments to structs, enums, traits and functions
• I have made corresponding changes to the documentation
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works

[lemunozm]

I remember something about this; thanks for fixing this @wischli !

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.71%. Comparing base (d3a0f0c) to head (8c1e0a5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1807   +/-   ##
=======================================
  Coverage   47.70%   47.71%           
=======================================
  Files         170      170           
  Lines       13576    13576           
=======================================
+ Hits         6477     6478    +1     
+ Misses       7099     7098    -1     

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[wischli]

I remember something about this; thanks for fixing this @wischli !

Yes you were not a fan of the reverted code path and thus right already back then 🫡