test: retry mops sources call in playground e2e tests

[ericswanson-dfinity]

Description

The mops sources command in the playground e2e tests fails often. The retry logic in the JS agent misses some classes of failures (basically, if fetch throws). Pending a fix for that, this change retries the mops sources call up to a few times, with simple/naive backoff.

Example test failures:

• https://github.com/dfinity/sdk/actions/runs/6114033428/job/16595080284#step:10:111
• https://github.com/dfinity/sdk/actions/runs/6090058722/job/16524749754#step:10:108
• https://github.com/dfinity/sdk/actions/runs/5929218365/job/16076870508#step:10:109
• https://github.com/dfinity/sdk/actions/runs/5858605218/job/15883717915#step:10:107
• https://github.com/dfinity/sdk/actions/runs/6316833146/job/17152786799?pr=3391

[chenyan-dfinity]

@ZenVoich Is this something you can fix on mops?

[ericswanson-dfinity]

@ZenVoich Is this something you can fix on mops?

See also dfinity/agent-js#774

[ZenVoich]

I also faced this problem in GitHub Actions in mops CI. I guess that's the IC's anti-DDoS in action.

I fixed this by caching mops packages, so if mops.toml has not changed mops will not re-download packages each time - https://github.com/ZenVoich/mops/blob/main/.github/workflows/ci.yml#L32-L37

I also published ic-mops 0.27.2 with reduced parallel downloading threads from 16 to 4 when mops install is running in GitHub Actions.