ci: multi-arch-build: run one job at a time

[a-gave]

Quick fix for Error: Action failed with "The process '/usr/bin/git' failed with exit code 1" caused by the fact that the next job does checkout before the previous one has pushed. It could be restructured as described here, but for now there is not much need, just do one at a time.

• https://github.com/peaceiris/actions-gh-pages/issues/608#issuecomment-834186136
• How to share matrix between jobs https://github.com/orgs/community/discussions/26284#discussioncomment-3251198

[codecov-commenter]

Codecov Report

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

Project coverage is 79.49%. Comparing base (4be8f34) to head (d33d4f7).

:exclamation: Current head d33d4f7 differs from pull request most recent head fea8ed2. Consider uploading reports for the commit fea8ed2 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1099 +/- ## ========================================== - Coverage 79.51% 79.49% -0.03% ========================================== Files 53 53 Lines 4599 4599 ========================================== - Hits 3657 3656 -1 - Misses 942 943 +1 ```

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[ilario]

One at a time seems enough :) Thanks!