Fix variant props of AlphaLoader component used in buttons (#2461)

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## Self Checklist

- [x] I wrote a PR title in **English** and added an appropriate
**label** to the PR.
- [x] I wrote the commit message in **English** and to follow [**the
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- [x] I [added the
**changeset**](https://github.com/changesets/changesets/blob/main/docs/adding-a-changeset.md)
about the changes that needed to be released. (or didn't have to)
- [x] I wrote or updated **documentation** related to the changes. (or
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## Related Issue

<!-- Please link to issue if one exists -->

<!-- Fixes #0000 -->

## Summary

<!-- Please brief explanation of the changes made -->

- 버튼 안에서 사용되는 로더 컴포넌트의 스타일을 버튼 컴포넌트 스펙에 맞게 수정했습니다. primary 버튼일 때는
on-overlay, 아닐 때는 secondary 여야 합니다.

## Details

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- 생략 

### Breaking change? (Yes/No)

<!-- If Yes, please describe the impact and migration path for users -->

- No

## References

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aware of -->

- [컴포넌트
스펙(internal)](https://www.notion.so/Button-95e7964c1e174aaa9923abc8ecc193b3?d=12874b55ec7c80dc9732001c0b74a23b&pvs=4#043567a82b2f4b5a9da4abd656d7ce08)

.changeset/cold-adults-thank.md

@@ -0,0 +1,5 @@
+---
+'@channel.io/bezier-react': patch
+---
+
+Fix `variant` props of `AlphaLoader` component used in `AlphaButton` component.

packages/bezier-react/src/components/AlphaButton/Button.tsx

@@ -125,7 +125,7 @@ export const Button = forwardRef<HTMLButtonElement, ButtonProps>(
             <AlphaLoader
               size="s"
               className={styles.Loader}
-              variant="on-overlay"
+              variant={variant === 'primary' ? 'on-overlay' : 'secondary'}
             />
           </div>
         )}

packages/bezier-react/src/components/AlphaFloatingButton/FloatingButton.tsx

@@ -124,7 +124,7 @@ export const FloatingButton = forwardRef<
           <AlphaLoader
             size="s"
             className={styles.Loader}
-            variant="on-overlay"
+            variant={variant === 'primary' ? 'on-overlay' : 'secondary'}
           />
         </div>
       )}

packages/bezier-react/src/components/AlphaFloatingIconButton/FloatingIconButton.tsx

@@ -79,7 +79,7 @@ export const FloatingIconButton = forwardRef<
           <AlphaLoader
             size="s"
             className={styles.Loader}
-            variant="on-overlay"
+            variant={variant === 'primary' ? 'on-overlay' : 'secondary'}
           />
         </div>
       )}

packages/bezier-react/src/components/AlphaIconButton/IconButton.tsx

@@ -83,7 +83,7 @@ export const IconButton = forwardRef<HTMLButtonElement, AlphaIconButtonProps>(
             <AlphaLoader
               size="s"
               className={styles.Loader}
-              variant="on-overlay"
+              variant={variant === 'primary' ? 'on-overlay' : 'secondary'}
             />
           </div>
         )}