CI: optimize caches used by CircleCI and GHA

[cce]

Summary

This updates the CircleCI and Github Actions caches to make it possible to find cache hits more often, by tweaking the key scheme for Go mod source and build caches.

For Github Actions, adding CI for merges to master, will ensure we are doing the same testing as on PRs, and also provide cached data on master that ensures the GHA cache will serve a hit from the master build for the first commit on a new branch PR.

A workflow can access and restore a cache created in the current branch, the base branch (including base branches of forked repositories), or the default branch (usually main). For example, a cache created on the default branch would be accessible from any pull request.
https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#restrictions-for-accessing-a-cache

Test Plan

Rerun this PR after changing some code.

[codecov]

Codecov Report

Merging #4461 (9e87a11) into master (3a2bcf4) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4461      +/-   ##
==========================================
+ Coverage   55.25%   55.26%   +0.01%     
==========================================
  Files         398      398              
  Lines       50265    50265              
==========================================
+ Hits        27772    27781       +9     
+ Misses      20174    20169       -5     
+ Partials     2319     2315       -4     

Impacted Files	Coverage Δ
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	⬇️
ledger/acctupdates.go	69.29% <0.00%> (-0.60%)	⬇️
catchup/service.go	69.38% <0.00%> (ø)
ledger/acctonline.go	78.36% <0.00%> (+0.52%)	⬆️
network/wsPeer.go	67.94% <0.00%> (+0.54%)	⬆️
data/transactions/verify/txn.go	44.64% <0.00%> (+0.89%)	⬆️
util/db/dbutil.go	48.48% <0.00%> (+4.24%)	⬆️
ledger/voters.go	73.13% <0.00%> (+4.47%)	⬆️

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[algolucky]

+1 for moving the caching into their own jobs.