Set JAVA_HOME on Windows

The `rJava` package needs the `$JAVA_HOME` variable to be built (and I think also at runtime). GHA runners have several Java versions but they do not set this variable by default.

I think the safest best is Java21 LTS which is available for both platforms. On intel runners:

```sh
JAVA_HOME=${JAVA_HOME_21_X64}
```

And on arm runners:

```
JAVA_HOME=${JAVA_HOME_21_AARCH64}
```

