test1143: disable MSYS2's POSIX path conversion

By default, the MSYS2 bash interprets http:/%HOSTIP:%HTTPPORT/want/1143
as a POSIX file list and converts it to a Windows file list.
Disable this with MSYS2_ARG_CONV_EXCL for the test to pass.

Ref https://github.com/msys2/msys2/wiki/Porting#filesystem-namespaces
Closes https://github.com/curl/curl/pull/2765