[vcpkg_from_github] Fix parse error on OSX

[JackBoosY]

When using --head, the github server will return the following json:

  {

    "ref": "refs/heads/master",
    "node_id": "MDM6UmVmMjg2MzkwOTpyZWZzL2hlYWRzL21hc3Rlcg==",
    "url": "https://api.github.com/repos/mosra/corrade/git/refs/heads/master",
    "object": {
      "sha": "bb626d650c7d76dd2846945ef22e9eb0003b6ea9",
      "type": "commit",
      "url": "https://api.github.com/repos/mosra/corrade/git/commits/bb626d650c7d76dd2846945ef22e9eb0003b6ea9"
    }

  }

There is a space after "sha":, causing the parsing to fail:

CMake Error at scripts/cmake/vcpkg_from_github.cmake:164 (message):
  Failed to parse API response from '':

So add parsing zero or more spaces after the :.

Fixes #20644.

[JackBoosY]

Related PR: #19815