ci: Setup daily longterm tests with longer epoch periods

[ptrus]

Fixes: #3301

This enables configuring epochtime length for the daily txsource test run.
To setup a daily run with longer epochs, setup another schedule in the existing pipeline and configure the epoch interval via the LONGTESTS_EPOCHTIME_INTERVAL env var.

TODO:

• test a long run with long epochs before setting it up
• Needs failure indicating commitments #3391 merged (as it fixes an edge case in the test)
• wait for one more test run: https://buildkite.com/oasisprotocol/oasis-core-long-tests/builds/242
• update the pipeline in buildkite after this is merged

buildkite-agent pipeline upload .buildkite/longtests.pipeline.yml -> .buildkite/longtests.pipeline.sh

[codecov]

Codecov Report

Merging #3322 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3322      +/-   ##
==========================================
- Coverage   66.17%   66.17%   -0.01%     
==========================================
  Files         371      371              
  Lines       33399    33399              
==========================================
- Hits        22102    22101       -1     
+ Misses       8056     8054       -2     
- Partials     3241     3244       +3     

Impacted Files	Coverage Δ
go/worker/compute/executor/committee/batch.go	66.66% <0.00%> (-16.67%)	⬇️
go/storage/mkvs/checkpoint/checkpointer.go	64.75% <0.00%> (-15.58%)	⬇️
go/storage/client/client.go	69.08% <0.00%> (-4.35%)	⬇️
go/consensus/tendermint/full/services.go	77.11% <0.00%> (-4.24%)	⬇️
go/storage/mkvs/insert.go	90.41% <0.00%> (-2.74%)	⬇️
go/storage/mkvs/lookup.go	72.63% <0.00%> (-2.11%)	⬇️
go/storage/mkvs/cache.go	78.40% <0.00%> (-1.88%)	⬇️
go/storage/mkvs/iterator.go	80.57% <0.00%> (-1.44%)	⬇️
go/runtime/transaction/transaction.go	72.36% <0.00%> (-1.32%)	⬇️
go/worker/compute/executor/committee/node.go	63.37% <0.00%> (-1.19%)	⬇️
... and 18 more

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