appveyor: Limit $PATHs for MinGW targeting builds
appveyor: Limit $PATHs for MinGW targeting builds

The toolchains should only need their system's executable paths,
and it is better to not make available tools not explicitly expected
(like python3 or unzip).

This should also help prevent "cygheap base mismatch detected" errors,
e.g. due to an unexpected version of unzip being on the path.

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