fix gradle caching (#4270)

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

## Description

https://github.com/smithy-lang/smithy-rs/pull/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.

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