Trim off spaces in `platform` string

This allows users to specify platform in a more humane way, for example:
```yaml
env:
  # equals to `linux/amd64, linux/arm/v6`
  platforms: >
    linux/amd64,
    linux/arm/v6

- uses: docker/setup-qemu-action@v2
  with:
    platforms: ${{ env.PLATFORMS }}
```

Signed-off-by: Chocobo1 <Chocobo1@users.noreply.github.com>

src/context.ts

@@ -8,6 +8,11 @@ export interface Inputs {
 export function getInputs(): Inputs {
   return {
     image: core.getInput('image') || 'tonistiigi/binfmt:latest',
-    platforms: core.getInput('platforms') || 'all'
+    platforms:
+      core
+        .getInput('platforms')
+        .split(',')
+        .map(v => v.trim())
+        .join(',') || 'all'
   };
 }