roachtest: jepsen tests pull code from github upon every run

[knz]

The jepsen roachtests do a git clone of the jepsen repo from Github upon every invocation.

Certain jepsen tests e.g. the network partition tests also do curl from github repos, e.g. to fetch toxiproxy-server

This is a broken design:

• it causes the test to fail upon a github temporary problem (eg #61930 #61928 #61927 #61925 #61923 #61922 #61918 #61969)
• it incurs egress costs for github (which is not friendly practice on the internet)
• it incurs ingress costs for us
• it makes the tests slightly slower on our side

Instead the test must be changed to use a local cache of the necessary source code.

This is a variation of #51543 and #52342. Meta-issue: https://github.com/cockroachdb/dev-inf/issues/39

Epic DEVINF-109

Jira issue: CRDB-2691

[knz]

cc @rickystewart for triage

[kenliu-crl]

reassigning to testeng

[blathers-crl[bot]]

cc @cockroachdb/test-eng