fix: flatten mutt ppc64el platform

[clwsgruk]

Fixes the broken platforms definition for ppc64el in snap/snapcraft.yaml.

Change:

• Drops the nested build-on clause from platforms.ppc64el, making it a flat map entry like the other architectures.

Verification:

• Parsed snap/snapcraft.yaml with Python/YAML.
• Asserted platforms.ppc64el is a flat key with no nested value.
• Asserted the platforms block contains no build-on clauses.
• Ran git diff --check.
• Commit is signed.

@jnsgruk please review.