do PR validation on Travis-CI (or GitHub Actions?) instead of Jenkins

[SethTisue]

the groundwork for this is already laid.

the main blocker had been that partest took too long but that's fixed now, so we'll be well under the 90 minute limit the Travis-CI folks have kindly granted us.

[SethTisue]

this is now pressing since scala-ci.typesafe.com is down.

WIP on Travis-based PR validation: scala/scala#6621

in order to finish this, we'll need to get scala-ci's Artifactory back up, for publishing the PR snapshots.

[SethTisue]

we'll need to get scala-ci's Artifactory back up, for publishing the PR snapshots

it's back up.

note that it isn't only about publishing the PR snapshots for public consumption, it's also about doing like the old validate/publish-core and validate/test scripts do, the so-called mini-bootstrap: we build, publish to Artifactory, point STARR to what we just built, then run the tests.

[SethTisue]

my initial stabs at this tried to share code with the full bootstrap-and-publish job, but the bootstrap stuff is so hairy (for both inherent and historical/accidental/fixable reasons), it's going to easier to work separately from that first, then once it's working, consider removing duplication. (Lukas agrees.)

the nicest experience will be achieved if we split up the parts of testAll so they all run in parallel on Travis and produces a separate logfile. but for starters I'm just trying to run testAll and get that working.

[SethTisue]

we can't publish the PR snapshots to Artifactory because pull request jobs on Travis-CI don't have access to secure environment variables

I suggest we:

• use publishLocal to do PR validation itself
• worry separately about publishing to Artifactory
  • probably by adding a new /publish command to Scabot (usable only by committers) that would push the PR branch to scala/scala (under some standardized naming scheme like PR/1234) at which point the existing publish stuff would take over. (this would have the advantage of publishing fully bootstrapped rather than mini-bootstrapped artifacts)

[SethTisue]

cleaner PR at scala/scala#6630. uses publishLocal for the mini-bootstrap; doesn't otherwise publish.