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Fix non-deterministic JDK preparation script causing cache misses (#23036)

The JDK preparation script (`jdk.sh`) embeds the output of `java
-version` as a comment. However, when Coursier downloads the JDK, it
outputs progress messages to stderr which get mixed with the actual
`java -version` output.

These progress messages are non-deterministic (contain timing info like
"Still downloading", "Downloaded", etc.) which causes the script content
to vary between runs, resulting in different cache keys for all JVM
compilation tasks.

This fix filters out Coursier download progress messages from the
captured stderr, keeping only the actual `java -version` output lines.

Impact: In testing, this reduced cache misses from 92% to 0.09% for JVM
builds across runs.

0 of 2 new or added lines in 1 file covered. (0.0%)

78764 of 98125 relevant lines covered (80.27%)

3.36 hits per line