GHA: fix caching old mingw-w64 toolchains in the Windows workflow

GHA: fix caching old mingw-w64 toolchains in the Windows workflow

- stop altering the `PATH` via `GITHUB_ENV`. This confused the
  `actions/cache` post-job, which needs to run in the exact same
  environment as its pre-job, to have a consistent cache entry "version"
  hash. Altering the `PATH` via `GITHUB_ENV` spills into the the
  post-job and breaks this hash. GHA doesn't reset the env automatically
  and I have not found a way to do it manually.

- add double-quotes where missing.

- move cache directory under `USERPROFILE` to not rely on absolute
  paths.

- make cache directory flatter and versionless.

Follow-up to 0914d8aadddac0d1459673d5b7f77e8f3378b22b #13759
Closes #13856