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.

This reverts commit 3ade5832.