Platform linux/arm64 fails for build cache

**Describe the bug**

The following task YAML fails:

```
version: v1.0.0
steps:
- build: -t samashahtest.azurecr.io/acb:arm64 --platform linux/arm64 https://github.com/Azure/acr-builder.git
cache: "enabled"
```

Fails with the error:

```
Error: failed to solve: rpc error: code = Unknown desc = failed to solve with frontend dockerfile.v0: failed to load LLB: runtime execution on platform linux/arm64 not supported
Usage:
/buildx build [OPTIONS] PATH | URL | - [flags]

```
**To Reproduce**
Steps to reproduce the behavior:
1.
2.
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**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**
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**Any relevant environment information**
- OS: [e.g. Windows]
- Version [e.g. v1.0.0]

**Additional context**
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