fix(@schematics/angular): use scss file extension instead of sass for styles.scss

[yjaaidi]

When using ng new --style=sass, ng-new schematic generates a styles.scss file but in build and test architects in angular.json references src/styles.sass.

This fix solves the mismatch issue for users generating an application with style option set to sass.

[clydin]

Thank you for the contribution. However, this is a duplicate of #13552

[yjaaidi]

D'OH! 😱
I don't understand how I missed the issue. I've looked for it though... but I don't remember what keywords I used 😅

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