chore: build: make the OhSnap epoch configurable by an envvar for devnets

[arajasek]

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Before you mark the PR ready for review, please make sure that:

• All commits have a clear commit message.
• The PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, INTERFACE BREAKING CHANGE, CONSENSUS BREAKING, build, chore, ci, docs,perf, refactor, revert, style, test
  • area: api, chain, state, vm, data transfer, market, mempool, message, block production, multisig, networking, paychan, proving, sealing, wallet, deps
• This PR has tests for new functionality or change in behaviour
• If new user-facing features are introduced, clear usage guidelines and / or documentation updates should be included in https://lotus.filecoin.io or Discussion Tutorials.
• CI is green

[codecov]

Codecov Report

Merging #7995 (0358ad8) into master (6080431) will decrease coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7995      +/-   ##
==========================================
- Coverage   39.26%   39.06%   -0.21%     
==========================================
  Files         660      660              
  Lines       71408    71408              
==========================================
- Hits        28041    27897     -144     
- Misses      38560    38690     +130     
- Partials     4807     4821      +14     

Impacted Files	Coverage Δ
chain/events/state/fastapi.go	75.00% <0.00%> (-25.00%)	⬇️
itests/kit/blockminer.go	73.52% <0.00%> (-17.65%)	⬇️
chain/events/message_cache.go	87.50% <0.00%> (-12.50%)	⬇️
node/impl/full/mpool.go	25.21% <0.00%> (-8.70%)	⬇️
chain/exchange/peer_tracker.go	66.66% <0.00%> (-8.61%)	⬇️
chain/exchange/client.go	49.15% <0.00%> (-8.48%)	⬇️
cli/util.go	41.66% <0.00%> (-8.34%)	⬇️
extern/storage-sealing/sector_state.go	92.30% <0.00%> (-7.70%)	⬇️
chain/events/observer.go	71.64% <0.00%> (-6.72%)	⬇️
markets/retrievaladapter/client_blockstore.go	62.50% <0.00%> (-6.25%)	⬇️
... and 26 more

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