Re-introduce informative CI jobs covering nodejs experimental versions

[rpl]

The CI job build-nodejs-experimental has been changed in #3014 to make it not strict on unit tests failures to unblock updating the CI job to remove the old nodejs 16 and add nodejs 20.

The failures currently hit by the job running on nodejs v21 looks very likely new issues with mocking ESM modules, and so it may be reasonale to assume they need to be fixed in the testdouble custom ESM module loader (as it was for the failures previous hit on nodejs v20 and fixed in testdouble 3.18.0 / quibble 0.7.0).

When we will bring a CI job for nodejs experimental versions we want to make it only informative (to let us prepare to investigate issues that are only hit on nodejs experimental versions while it has not reached stability yet) and not blocking releasing new web-ext versions.

Circle CI doesn't seem to include any feature to allow failing tests to be reported as warnings, and so in the short term we kept the experimental job but made it less strict, that isn't great because the failures would not be visible unless we explicitly look to the CI job logs and so when we will look into bringing back a CI job for the nodejs experimental versions we want to consider other options that may improve the visibility of those failures without making them blocking as it used to be (e.g. we could consider introducing a github action job for the nodejs experimental versions).