ref(CI): Speed up testing in GHA

[lobsterkatie]

This PR makes a number of changes to our build and test process on CI, with the goal of markedly speeding up our GHA workflows.

Notable Changes

• In our GHA workflow, dependencies and built packages are now cached separately, since the former rarely change and the latter change with each commit.
• Ember tests now run separately from other tests. They are the slowest part of our test suite, and this allows for parallelization.
• Just because it's nice, all GHA steps now have names.
• There is now the ability to skip certain tests when running legacy Node versions. We lose the benefit of those tests in those cases (boo), but gain the benefit of being able to test the rest of the package, which we previously weren't able to (yay).
• The test script used by our matrix of Node version tests has been cleaned up, primarily to remove redundant calls to yarn install and yarn build (since both of those are now handled by the GHA workflow), but also to simply clear out old cruft.

Results

Prior to this PR, CI has been taking around 27 minutes, give or take:

After this PR, Ci takes around 20 minutes if third-party dependencies have changed (not frequent), and a little over 18 minutes if they haven't (much more common):

Follow Up

According to @k-fish, we may be able to gain a little more time by reducing the Ember matrix a bit.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	20.38 KB (0%)
@sentry/browser - Webpack	21.27 KB (0%)
@sentry/react - Webpack	21.29 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	27.32 KB (0%)

[rhcarvalho]

Nicely done!

[HazAT]

I am going to merge this to prepare a release