Pin Rust/Java versions, cache Gradle dependencies in CI
requested to merge SalvatoreT/uniffi-kotlin-multiplatform-bindings:salvatoret/19/improve-ci-build into main
Make sure the Rust and Java versions stay stable so that folks running pipelines get the same results.
Pre-run the Gradle command so the ~/.konan (Kotlin Native),
~/.gradle, /usr/local/cargo/, and /usr/local/rustup/ caches are
all warm.
I updated the pipelines to (1) only build the Docker image if a new one is needed but also (2) make a way to manually build a new image.
Closes #19 (closed)