Switch our workflows to ubuntu 20.04

[jepler]

This switches some which were using ubuntu-latest and some which were using ubuntu-16.04, all to 20.04. When 22.04 is added on GH Actions we can revisit this.

[jepler]

The lone failure was a network glitch. However, as this PR's home branch is not adafruit/circuitpython, it doesn't actually test s3 uploads which @dhalbert mentioned as a potential incompatibility.

[dhalbert]

I looked over the aws changes, and I just see this as a possible issue:
https://docs.aws.amazon.com/cli/latest/userguide/cliv2-migration.html#cliv2-migration-output-pager

Looks like we should set AWS_PAGER to nothing to avoid having it hang waiting for paging. (I'm surprised that a pager might not notice it's being run in a script and do nothing.)

[dhalbert]

These steps use ubuntu-latest instead of an explicit version, so maybe we should change them to ubuntu-20.04. That way they won't change out from under us in two years.

workflows/create_website_pr.yml
13:    runs-on: ubuntu-latest

workflows/build.yml
418:    runs-on: ubuntu-latest

workflows/pre-commit.yml
14:    runs-on: ubuntu-latest

[jepler]

I had a mistaken commit in there (disabling ulab on some stm boards) so I rebased it out and force-pushed.

[dhalbert]

I just remembered this: #3629 (review)
@jepler says:

turns out that when the branch is made in adafruit/circuitpython instead of jepler/circuitpython the upload steps can be tested. Nice, I didn't realize that.

[jepler]

I pushed it to an adafruit branch as well, run results will be at https://github.com/adafruit/circuitpython/actions/runs/368709935

[dhalbert]

Uploads in adafruit branch build look fine! Thanks for doing this!

[jepler]

@dhalbert excellent. Looks like you have to re-review since I had entered this in draft state

[dhalbert]

I think it's not green just because of the one failing build (which is just network trouble).