Add ability to install K3s PR Artifact from GitHub

[dereknola]

Proposed Changes

Context: With the removal of AWS S3 uploads for K3s PRs, we need another way for QA to easily validate open PR builds of K3s.

• Adds the ability for the install script to pull the K3s executable built during GitHub CI (specifically the integration test build step found here)

• Requires the use of a GITHUB_TOKEN with K3s-io/k3s repo permissions. This will limit the use of this feature to any authorized QA and Dev. Unfortunately, this is a requirement around downloading GitHub artifacts.

Note: Only this PR has the changes necessary to support "installing from PR". This is due to the sha256sum for the k3s binary being added as a GitHub artifact in this PR. So this is a "PRs newer than this one" feature, not "works on all PRs going back forever" feature.

Types of Changes

Install script feature

Verification

Testing

Covered by existing install tests

export GITHUB_TOKEN=<XXX>
cd tests/install/ubuntu-focal/
INSTALL_K3S_PR=9185 vagrant up

Linked Issues

User-Facing Change

Adds `INSTALL_K3S_PR` option to install a build of K3s from any open PR with CI approval

Further Comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d8907ce) 40.53% compared to head (30ec9a9) 40.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9185      +/-   ##
==========================================
- Coverage   40.53%   40.52%   -0.01%     
==========================================
  Files         154      154              
  Lines       16555    16555              
==========================================
- Hits         6710     6709       -1     
- Misses       8697     8698       +1     
  Partials     1148     1148              

Flag	Coverage Δ
inttests	37.68% <ø> (-0.01%)	⬇️
unittests	14.53% <ø> (ø)

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