Fix vcpkg_install.bat (git-for-windows#6298)

While researching a build failure in `shears/seen`, I noticed that the
`vcpkg` Pipeline [has been broken since August last
year](https://dev.azure.com/git/git/_build?definitionId=9&_a=summary) 🤦.
The reason is that the `schannel` feature of `curl` was removed, as part
of microsoft/vcpkg#46459. This PR did not remove
_Secure Channel support_, it merely removed the ability to specify it
_as a feature_. Let's accommodate for that.

Author: dscho

compat/vcbuild/vcpkg_install.bat

@@ -127,5 +127,5 @@ set features=
 goto :EOF
 
 :curl_features
-set features=[core,openssl,schannel]
+set features=[core,ssl]
 goto :EOF