fix gradle caching (#4270)

## Motivation and Context
Still seeing intermittent transient failures due to Gradle wrapper
failing to download

## Description

#4217 was meant to fix
this, I think we just missed that the installation and wrapper is being
downloaded to an intermediate image rather than the final image. The
final image did not have the gradle wrapper binary nor the env variable
related to it set.

```
> env | sort
...
GRADLE_USER_HOME=/home/build/.gradle
...

> [localbuild@047e7e4b0e64 build]$ ls -lsa /home/build/.gradle
total 40
4 drwxr-xr-x. 10 build build 4096 Aug 15 14:39 .
4 drwxrwxr-x.  1 build build 4096 Aug 15 14:54 ..
4 drwxr-xr-x.  7 build build 4096 Aug 15 14:38 caches
4 drwxr-xr-x.  3 build build 4096 Aug 15 14:38 daemon
4 drwxr-xr-x.  2 build build 4096 Aug 15 14:38 jdks
4 drwxr-xr-x.  2 build build 4096 Aug 15 14:39 kotlin-profile
4 drwxr-xr-x.  5 build build 4096 Aug 15 14:38 native
4 drwxr-xr-x.  3 build build 4096 Aug 15 14:38 notifications
4 drwxr-xr-x.  3 build build 4096 Aug 15 14:39 .tmp
4 drwxr-xr-x.  3 build build 4096 Aug 15 14:38 wrapper
```

## Testing
Build image locally and see that gradle wrapper is present and
GRADLE_USER_HOME is set.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Author: aajtodd

tools/ci-build/Dockerfile

@@ -90,6 +90,7 @@ RUN set -eux; \
 
 FROM install_rust AS local_tools
 ARG rust_nightly_version
+ENV GRADLE_USER_HOME=/home/build/.gradle
 COPY . tools/ci-build
 # when `checkout_smithy_rs_tools` is set to true, this commit will be checked out
 ARG smithy_rs_commit_hash=main
@@ -229,13 +230,15 @@ COPY --chown=build:build --from=local_tools /opt/cargo/bin/mdbook /opt/cargo/bin
 COPY --chown=build:build --from=local_tools /opt/cargo/bin/mdbook-mermaid /opt/cargo/bin/mdbook-mermaid
 COPY --chown=build:build --from=nodejs /opt/nodejs /opt/nodejs
 COPY --chown=build:build --from=musl_toolchain /usr/local/musl/ /usr/local/musl/
+COPY --chown=build:build --from=local_tools /home/build/.gradle /home/build/.gradle
 ENV PATH=/opt/nodejs/bin:/opt/cargo/bin:$PATH:/usr/local/musl/bin/ \
     NODE_HOME=/opt/nodejs \
     CARGO_HOME=/opt/cargo \
     RUSTUP_HOME=/opt/rustup \
     RUST_STABLE_VERSION=${rust_stable_version} \
     RUST_NIGHTLY_VERSION=${rust_nightly_version} \
     CARGO_INCREMENTAL=0 \
+    GRADLE_USER_HOME=/home/build/.gradle \
     LANG=en_US.UTF-8 \
     LC_ALL=en_US.UTF-8
 # SMITHY_RS_DOCKER_BUILD_IMAGE indicates to build scripts that they are being built inside of the Docker build image.