[Checkbox] Update icon hard coded color value (#10999)

### WHY are these changes introduced?

Fixes Shopify/polaris-internal#1082

### WHAT is this pull request doing?

Removes hard coded value and replaces it with tokens

.changeset/moody-scissors-greet.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Updated `Checkbox` icon to use tokens vs hard coded value

polaris-react/src/components/Checkbox/Checkbox.scss

@@ -62,6 +62,11 @@
         transform var(--p-motion-duration-150) var(--p-motion-ease-out);
       opacity: 1;
 
+      /* stylelint-disable-next-line selector-max-combinators -- need to target svg from icons package */
+      svg {
+        fill: var(--p-color-text-brand-on-bg-fill);
+      }
+
       &.animated {
         transition: initial;
       }
@@ -86,7 +91,7 @@
 
     // stylelint-disable-next-line selector-max-combinators -- target disabled icon color
     ~ .Icon svg {
-      fill: var(--p-color-checkbox-icon-disabled);
+      color: var(--p-color-checkbox-icon-disabled);
     }
   }
 
@@ -141,7 +146,7 @@
   }
 
   svg {
-    fill: var(--p-color-text-brand-on-bg-fill);
+    color: var(--p-color-text-brand-on-bg-fill);
 
     position: absolute;
     top: 0;
@@ -158,7 +163,7 @@
 // stylelint-disable selector-max-specificity, selector-max-class, selector-max-combinators, selector-max-compound-selectors -- generated by polaris-migrator DO NOT COPY
 .error {
   .Icon svg {
-    fill: var(--p-color-text-critical-on-bg-fill);
+    color: var(--p-color-text-critical-on-bg-fill);
   }
 
   .Input {

polaris-react/src/components/Checkbox/Checkbox.tsx

@@ -141,7 +141,7 @@ export const Checkbox = forwardRef<CheckboxHandles, CheckboxProps>(
           transform="translate(2 2.980376)"
           opacity="0"
           fill="none"
-          stroke="#fff"
+          stroke="currentColor"
           strokeWidth="2"
           strokeLinecap="round"
           strokeLinejoin="round"