chore: only run jobs in packages that have changed since master

[achingbrain]

Should make PR builds much faster. Might have to be a bit smarter about coverage reports over time though.

[hugomrdias]

Can you explain further this change, maybe an example.
Doesn't lerna run tests only in packages with changed files?

[achingbrain]

Doesn't lerna run tests only in packages with changed files?

Not by default - unlike releases (which use lerna updated) you have to specify --since [ref] for that.

For example #2914 only changed an example, which nothing depends on so only the tests for the example should have run, yet in the build for that PR it ran everything - interop, interface tests, the lot.

Interface & interop tests are defined in the ipfs and ipfs-http-client packages, which were not changed so should not have run.

When specifying --since to lerna run it does still work out the dependency graph and run scripts in packages that haven't changed but are dependant on ones that have.