Fix proxy-exec store temporary directory

Arran Walkerrequested to merge
ajwalker/fix-proxy-exec-store-tmp-dir into main

What does this MR do?

Fixes the proxy-exec store directory.

Why was this MR needed?

We can't rely on the OS temp path across build stages in container environments.

What's the best way to test this MR?

I've added an integration test.

What are the relevant issue numbers?

Edited by Arran Walker

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