ci: fix checkout action to checkout branch at current state

[wschurman]

Why

I just wasted ~2 hours on this, so I bet it'll affect others as well.

1. Create branch A at base T, add some commits, create PR(A). See checkout passes and no conflicts.
2. Create branch B at base T, add some commits that create logical conflict with branch A, create PR(B). See checkout passes and no conflicts.
3. Land PR(B) into master.
4. Push additional commit only changing comments to branch A to address comments on PR(A). See that the build is now failing in CI. Run the build locally on branch A and see that it is passing. Waste 2 hours only to find out that instead of running the build on branch A as is represented in PR(A), the "checkout" action doesn't actually "checkout" branch. Instead it creates a merge commit of all the commits on the branch into the current state of the destination branch, which is super non-obvious and not the matching behavior of CircleCI or other CI platforms which checkout the branch ref. The action should be called "checkout-base-and-create-merge-commit".

actions/checkout#27

Adding @nicknovitski as reviewer since he probably is most familiar with how this could affect us in other places that use GH actions.

How

https://github.com/actions/checkout#Checkout-pull-request-HEAD-commit-instead-of-merge-commit

Test Plan

Wait for build.

[codecov]

Codecov Report

Merging #55 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   93.95%   93.95%           
=======================================
  Files          57       57           
  Lines        1357     1357           
  Branches      147      147           
=======================================
  Hits         1275     1275           
  Misses         81       81           
  Partials        1        1           

Flag	Coverage Δ
#integration	93.95% <ø> (ø)
#unittest	93.95% <ø> (ø)

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[jkhales]

Very weird and not obvious.

[mcg1969]

Glad to find this after some googling