fix(maker-snap): allow MakerSnapConfig to also be SnapcraftConfig

[malept]

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Try to make it more obvious where to put custom snapcraft.yaml config.

[codecov]

Codecov Report

Merging #1725 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1725   +/-   ##
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  Coverage   70.26%   70.26%           
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  Files          74       74           
  Lines        2189     2189           
  Branches      412      412           
=======================================
  Hits         1538     1538           
  Misses        465      465           
  Partials      186      186           

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[RobbieTheWagner]

Are there any docs on how to use a custom config?