ci: modification of main.yml to use cmake for vs-build job

ci: modification of main.yml to use cmake for vs-build job

Teach .github/workflows/main.yml to use CMake for VS builds.

Modified the vs-test step to match windows-test step. This speeds
up the vs-test. Calling git-cmd from powershell and then calling git-bash
to perform the tests slows things down(factor of about 6). So git-bash
is directly called from powershell to perform the tests using prove.

NOTE: Since GitHub keeps the same directory for each job
(with respect to path) absolute paths are used in the bin-wrapper
scripts.

GitHub has switched to CMake 3.17.1 which changed the behaviour of
FindCURL module. An extra definition (-DCURL_NO_CURL_CMAKE=ON) has been
added to revert to the old behaviour.

In the configuration phase CMake looks for the required libraries for
building git (eg zlib,libiconv). So we extract the libraries before we
configure.

To check for ICONV_OMITS_BOM libiconv.dll needs to be in the working
directory of script or path. So we copy the dlls before we configure.

Signed-off-by: Sibi Siddharthan <sibisiddharthan.github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>