meta: use build-for in snap.yaml architecture

[mr-cal]

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Details

Previously, using build-for: all in a snapcraft project would cause meta/snap.yaml to contain the build-on architecture, due to this line from my first implementation of adding support for the architectures keyword.

Now, build-for: all will cause meta/snap.yaml to use architectures: all.

Source

https://bugs.launchpad.net/snapcraft/+bug/1999580
https://forum.snapcraft.io/t/incorrect-architecture-with-snapcraft-7-x-and-build-for-build-on/34130
(CRAFT-1535)

[codecov-commenter]

Codecov Report

Merging #4150 (df68e51) into main (6f62368) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head df68e51 differs from pull request most recent head 4645c97. Consider uploading reports for the commit 4645c97 to get more accurate results

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@@            Coverage Diff             @@
##             main    #4150      +/-   ##
==========================================
+ Coverage   94.80%   94.84%   +0.03%     
==========================================
  Files         648      648              
  Lines       54280    54292      +12     
==========================================
+ Hits        51461    51494      +33     
+ Misses       2819     2798      -21     

Impacted Files	Coverage Δ
snapcraft/parts/lifecycle.py	92.90% <ø> (+4.19%)	⬆️
tests/unit/parts/test_lifecycle.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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