Icons resolved from icon theme still prints error messages

[lunacd]

Check existing issues

• I've verified that this bug isn't described by any existing issues.

Bug description

In this PR, this copilot review seems legit. After the icon is successfully resolved from icon theme, icon local variable is not updated and the subsequent check still fails.

Steps to reproduce

• Create snapcraft.yaml that produces $CRAFT_PRIME/usr/share/icons/hicolor/256x256/app/example.png
• Create a desktop file with Icon=example
• Run snapcraft pack
• Observe that it incorrectly prints

Icon 'example' specified in desktop file 'usr/share/applications/example.desktop' not found in prime directory.

[mr-cal]

FYI @soumyaDghosh

[soumyaDghosh]

I think the problem lies in a different place. I have found the solution, but I'd like to add some tests to catch it as well. The unit tests should have been able to catch this issue. Thanks a lot @lunacd for flagging this