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.

Co-authored-by: Alexey Tsvetkov <alexey.tsvetkov@jetbrains.com>

It is useful to have two separate default values: one for local development and one for CI.
This will allow us to advance published version without
touching CI configuration.

They don't work, because they expect gradle/wrapper/gradle-wrapper.jar to exist.
That directory was moved to skiko/ directory.

This mainly affects running tasks from Intellij,
where changing JAVA_HOME can be inconvenient.

In command-line `./gradlew` Gradle JVM is equal to JAVA_HOME by default

load with arguments was removed in https://github.com/JetBrains/skija/commit/70541e56916fef49e7e35e013a2855f64a6fd2e7

This allows to open root as Intellij project,
then import "skiko" as Gradle project.