Pip for ci

[cisaacstern]

Towards #604

This PR tries just using pip for testing, with goal of:

• simplifying workflow files
• speeding up test runtimes
• making testing install more closely reflect users' installs

[cisaacstern]

This seems like an overall win to me and an important, albeit incremental, improvement in test runtime. Specifically I view this as a:

• Big improvement in intelligibility and maintainability of the test workflow. This is a lot simpler. Among other things, the number of checks per commit is reduced by half (because each check previously had a prepare-env step which is now removed), which is big win for readability of the checks results which were getting quite cluttered.
• Big improvement in degree to which our test environment replicates users pip installed environments. This PR surfaced two big discoveries in that realm: (1) We had not specified apache-beam as a dependency 😱 ; and (2) GRIB pipelines should run in conda environments (due to the dependency on cfgrib), but other than that, everything here works from pip.
• Incremental improvement in test run time. My anecdotal comparison to checks run times on some other open PRs is that we might save about 1 minute of the ~11-15 minute test running time. That's not nothing, but bigger improvements will probably come with reducing fixture redundancy.

The simplification and user environment replication alone is worth it here, IMO, and the marginal speed up is a nice add-on. Before merging noting a few future directions for follow-on work:

• Restore GRIB testing with a separate requires-conda test setup somewhere
• Catching issues like Workaround broken conda release of beam 2.50.0 #603 is nice, so we should generally try to test against conda environments somewhere, but maybe on conda-forge/pangeo-forge-recipes-feedstock?
• It's good to have the upstream-dev testing (and we should probably include Zarr-python there) but it's a big heavy to run on every commit. Maybe we can move that to nightly runs on a chron schedule?

Edit: Moved these points ☝️ to #606