tech-debt: fix deprecated warnings in build script

[agliga]

Fix these

Deprecation Warning: Sass's behavior for declarations that appear after nested
rules will be changing to match the behavior specified by CSS in an upcoming
version. To keep the existing behavior, move the declaration above the nested
rule. To opt into the new behavior, wrap the declaration in `& {}`.

More info: https://sass-lang.com/d/mixed-decls

   ┌──> src/sass/button/button.scss
10 │       border-radius: var(--btn-border-radius, calc($button-height-regular / 2));
   │       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ declaration
   ╵
   ┌──> src/sass/mixins/private/_button-mixins.scss
21 │ ┌     &--fixed-height {
22 │ │         height: 40px;
23 │ │     }
   │ └─── nested rule
   ╵
    src/sass/button/button.scss 10:5         @import
    src/sass/bundles/skin-headless.scss 9:9  @import
    src/sass/bundles/skin-full.scss 10:9     @import
    src/routes/+layout.style.scss 9:9        root stylesheet

Deprecation Warning: Sass's behavior for declarations that appear after nested
rules will be changing to match the behavior specified by CSS in an upcoming
version. To keep the existing behavior, move the declaration above the nested
rule. To opt into the new behavior, wrap the declaration in `& {}`.

More info: https://sass-lang.com/d/mixed-decls

   ┌──> src/sass/button/button.scss
10 │       border-radius: var(--btn-border-radius, calc($button-height-regular / 2));
   │       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ declaration
   ╵
   ┌──> src/sass/mixins/private/_button-mixins.scss
25 │ ┌     &--truncated {
26 │ │         @include btn-truncate();
27 │ │
28 │ │         height: 40px;
29 │ │     }