Tweaks.

CI publishing for all platforms

Until the commit https://github.com/google/skia/commit/0d6f81593b1fa222e8e4afb56cc961ce8c9be375
is included in the version of Skia we use, the build fails with modern versions
of Visual Studio Build Tools.

As a temporary workaround, this commit adds the necessary include after Skia checkout.

Sometimes Gradle C++ plugin fails to find Visual Studio Build Tools
automatically (see https://github.com/gradle/gradle-native/issues/617).

This commit adds optional environment variable for providing
VSBT path manually.

Use -Pdeploy.release=true to override

This commit gets rid of bash scripts. Everything is done by Gradle.
Also setting SKIA_DIR became optional:
SKIA_DIR is used, when it is set,
otherwise the binaries are downloaded from bintray
(see gradle.properties for the exact artifact pattern).

This change simplifies building on Windows.