refactor: remove tmpDir management and buildx installation

Since setup-docker-builder handles all infrastructure setup, build-push-action
no longer needs to:
- Install buildx (just assert it's available)
- Manage temporary directories (handled by actions toolkit)

Changes:
- Replace buildx installation with simple availability assertion
- Remove tmpDir state management entirely
- Remove buildx-version input parameter
- Clean up unused imports and functions

The action now assumes buildx is already configured by setup-docker-builder
or another setup action, making it simpler and more focused.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/context.ts

@@ -58,7 +58,6 @@ export interface Inputs {
   ulimit: string[];
   'github-token': string;
   nofallback: boolean;
-  'buildx-version': string;
 }
 
 export async function getInputs(): Promise<Inputs> {
@@ -96,8 +95,7 @@ export async function getInputs(): Promise<Inputs> {
     target: core.getInput('target'),
     ulimit: Util.getInputList('ulimit', {ignoreComma: true}),
     'github-token': core.getInput('github-token'),
-    nofallback: core.getBooleanInput('nofallback'),
-    'buildx-version': core.getInput('buildx-version')
+    nofallback: core.getBooleanInput('nofallback')
   };
 }