Use env variables instead of hardcoded paths for ms-mpi search

Proposed changes

Microsoft MPI Redist and SDK installers set environment variables (MSMPI_BIN, MSMPI_INC, MSMPI_LIB32, MSMPI_LIB64) that described in the example in Microsoft MPI repository. These environment variables are widely used. For example, CMake uses them in it's FindMPI module there and there, Meson also uses these variables to locate MS-MPI.

This PR changes the way MS-MPI is searched. Instead of hardcoded paths to MS-MPI Redist and SDK, paths from environment variables mentioned above are used. This allows to use MS-MPI Redist and SDK installed to custom path.

I tested these changes with the latest MS-MPI Redist and SDK installed to the default path and to custom path.

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[x] Bugfix (non-breaking change which fixes an issue)
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[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
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[ ] Refactoring (no functional changes, no api changes)
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Checklist

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bfgroup / b2