Clean secp256k1 build artifacts before build

[awesome-doge]

Summary

This PR fixes CI build failures by cleaning the third-party/secp256k1 directory before starting the build process.

• Adds a cleanup step to remove stale autotools artifacts from secp256k1 submodule
• Uses git clean -fdx and git checkout . to ensure a clean build state
• Prevents build failures caused by leftover files from previous builds

Problem

The CI build occasionally fails due to stale autotools-generated files in the secp256k1 submodule that conflict with fresh builds.

Solution

Added a new step "Clean third-party build artifacts" that runs before the build step to:

1. Navigate to third-party/secp256k1
2. Remove all untracked files with git clean -fdx
3. Reset any modified files with git checkout .

[DanShaders]

The CI build occasionally fails due to stale autotools-generated files in the secp256k1 submodule that conflict with fresh builds.

Can you link an example workflow? I never seen this particular flake.

Invalidate ccache to fix illegal instruction error

Thanks for debugging this! Can you please drop the commit though (I want to do a more comprehensive fix)?