fix(colors): update classic sass file variable initialization

[joshblack]

Closes #8565

This updates our color build step to emit all variables before the mixins in the file.

Changelog

New

Changed

• Update the colors build step to emit all variables before the mixins in the file

Removed

Testing / Reviewing

• Pull down the PR and run yarn build in packages/colors
• Inspect scss/_mixins.scss
• Verify that all color variables are declared at the start of the file that are included in the ibm--colors and carbon--colors mixins
• Verify that all variables defined in the mixins use the !global flag

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[tw15egan]

Everything building as expected 👍🏻 ✅

[andreancardona]

also built as expected!