ci: optimise arm workflow

[skl]

This PR:

• Re-uses the integration-matrix sharding, and shares the makefile target between vm and arm workflows as they both run 5 partitions of "TestMultiProcess"
• Use gotestsum with auto-retry

This should improve test isolation, reduce the likelihood of transient port conflicts and other errors, and also reduce the re-run time for failed workflows.

ARM workflow wall clock: from ~14m to ~9m.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.39%. Comparing base (ede9606) to head (7ccd16a).
⚠️ Report is 5 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (ede9606) and HEAD (7ccd16a). Click for more details.

HEAD has 29 uploads less than BASE

Flag	BASE (ede9606)	HEAD (7ccd16a)
oats-test	5	0
k8s-integration-test	15	0
integration-test	9	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1320      +/-   ##
==========================================
- Coverage   43.52%   37.39%   -6.13%     
==========================================
  Files         305      305              
  Lines       32864    32864              
==========================================
- Hits        14305    12291    -2014     
- Misses      17639    19839    +2200     
+ Partials      920      734     -186     

Flag	Coverage Δ
integration-test	?
integration-test-arm	0.00% <ø> (ø)
integration-test-vm-x86_64-5.15.152	0.00% <ø> (?)
integration-test-vm-x86_64-6.10.6	0.00% <ø> (?)
k8s-integration-test	?
oats-test	?
unittests	44.30% <ø> (ø)

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