ci: make manifest publishing run in serial

[rfratto]

It looks like there's some issues with publishing the Docker manifest after a merge. This PR does two things:

1. Make the CI fail if not all images in the manifest get published. This will be annoying but leads to less confusion.
2. Runs the manifest steps serially. Running them in parallel might have been contributing to the manifest steps flaking before.

[codecov-io]

Codecov Report

Merging #1633 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1633      +/-   ##
==========================================
- Coverage   61.87%   61.84%   -0.03%     
==========================================
  Files         109      109              
  Lines        8304     8304              
==========================================
- Hits         5138     5136       -2     
- Misses       2773     2774       +1     
- Partials      393      394       +1

Impacted Files	Coverage Δ
pkg/ingester/transfer.go	65% <0%> (-1.43%)	⬇️
pkg/logql/evaluator.go	91.75% <0%> (-1.04%)	⬇️
pkg/promtail/targets/tailer.go	78.16% <0%> (+2.29%)	⬆️

[slim-bean]

let's give it a try!