Build PRs using artifact registry packages

[Polber]

This PR configures maven on GitHub Actions workers (self-hosted) to pull dependencies from a Google-hosted artifact registry. This ensures that PR's are able to stage templates the same way that templates are staged during the release process to detect possible package discrepancies earlier.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 45.08%. Comparing base (b72eaa2) to head (04bc025). Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1862      +/-   ##
============================================
- Coverage     45.09%   45.08%   -0.01%     
+ Complexity     3935     3609     -326     
============================================
  Files           835      835              
  Lines         49703    49703              
  Branches       5232     5232              
============================================
- Hits          22412    22411       -1     
  Misses        25625    25625              
- Partials       1666     1667       +1     

Components	Coverage Δ
spanner-templates	66.26% <ø> (-0.01%)	:arrow_down:
spanner-import-export	64.18% <ø> (-0.03%)	:arrow_down:
spanner-live-forward-migration	76.18% <ø> (ø)
spanner-live-reverse-replication	76.54% <ø> (ø)
spanner-bulk-migration	85.57% <ø> (ø)

see 1 file with indirect coverage changes

[damccorm]

Could you add a description of what/why this is doing now that we pulled out the urgent pieces? It does also look like checks might be persistently failing (not sure if that's a flake or not)

[Polber]

@damccorm It's also hard to test because new caches get created by other workflows which affect this one being able to use AR packages instead of maven central. Once it is merged, I'll be able to clear caches and all new ones will use AR packages.

I'm pretty confident this change works, but let's wait until next week to merge so I can keep an eye on actions and see if anything fails.