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.

src/test/perl/PostgreSQL/Test/Utils.pm

@@ -142,7 +142,8 @@ BEGIN
 	# Must be set early
 	$windows_os = $Config{osname} eq 'MSWin32' || $Config{osname} eq 'msys';
 	# Check if this environment is MSYS2.
-	$is_msys2 = $^O eq 'msys' && `uname -or` =~ /^[2-9].*Msys/;
+	$is_msys2 = $windows_os && -x '/usr/bin/uname'  &&
+	  `uname -or` =~ /^[2-9].*Msys/;
 
 	if ($windows_os)
 	{