pgsql: Improve msys2 detection for TAP tests

Improve msys2 detection for TAP tests

Perl instances on some msys toolchains (e.g. UCRT64) have their
configured osname set to 'MSWin32' rather than 'msys'. The test for
the msys2 platform is adjusted accordingly.

Backpatch to release 14.

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/999dc1d2650883b817ea6d4e393ca8ba84a8687d

Modified Files
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src/test/perl/TestLib.pm | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)