CI: Run Windows build nightly to speed up PR CI builds

[michaeldiamant]

Runs build-test-windows nightly instead of on each PR commit in order to speed up PR CI builds.

Presuming we merge #4427, build-test-windows becomes the bottleneck. We estimate #4427 reduces build time to ~15min while build-test-windows takes ~25min.

Since we recently migrated Mac builds to nightly, the PR proposes a similar treatment for Windows. I'm unfamiliar with the history here and am opening the PR to gauge comfort levels.

[michaeldiamant]

Notes:

• I'll leave the PR as a draft while confirming the scheduled job starts as intended.
• It seems .github/workflows/build.yml already defines Slack notification for build failures. For the moment, I'm trusting it works and requires no modification.

[codecov]

Codecov Report

Merging #4432 (89223f7) into master (0b9580b) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4432      +/-   ##
==========================================
- Coverage   55.24%   55.22%   -0.02%     
==========================================
  Files         398      398              
  Lines       50148    50148              
==========================================
- Hits        27706    27696      -10     
- Misses      20133    20137       +4     
- Partials     2309     2315       +6     

Impacted Files	Coverage Δ
crypto/merkletrie/trie.go	66.42% <0.00%> (-2.19%)	⬇️
agreement/cryptoVerifier.go	67.60% <0.00%> (-2.12%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
crypto/merkletrie/node.go	91.62% <0.00%> (-1.87%)	⬇️
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
ledger/tracker.go	73.93% <0.00%> (-0.86%)	⬇️
ledger/acctonline.go	78.36% <0.00%> (-0.53%)	⬇️
ledger/accountdb.go	72.58% <0.00%> (-0.08%)	⬇️
network/wsNetwork.go	64.82% <0.00%> (+0.19%)	⬆️
catchup/service.go	68.88% <0.00%> (+0.24%)	⬆️
... and 3 more

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[michaeldiamant]

Marking ready for review because I don't think there exists a way to execute as cron from the non-default branch. At least not without having run once from the default branch.

[michaeldiamant]

Closing - Since #4443 reduces Windows build time to the point where it's no longer the bottleneck, let's close the PR. Can revisit the situation should Windows build time become the bottleneck.