Validate installed sbt on disk; split cache restore/save

[arashi01]

Problem

Windows CI intermittently fails at Setup PATH:

Cache restored from key: Windows-sbt-2.0.2-1.1.25
...
Run cd "$SBT_TOOLPATH"
ls: cannot access 'sbt/bin/sbt': No such file or directory
Error: Process completed with exit code 2.

The distribution cache reported a hit, so Download and Install (gated on cache-dir.outputs.cache-hit) was skipped - but the restored blob had no sbt/bin/sbt. Deleting the cache only unblocks temporarily; the poisoned blob comes back; needing a manual purge via gh to unblock/fix the build.

Changes

• Gate the install on the sbt binary on disk, not cache-hit: Download and Install self-checks sbt/bin/sbt and re-downloads on a miss or a corrupt restore.
• Assert sbt/bin/sbt after unzip so a partial extraction fails at the install, not at Setup PATH.
• Split the distribution actions/cache into restore + save; save runs only after a verified install, so an incomplete $SBT_TOOLPATH is never cached and the auto post-job save that created the issue is gone.
• Bump SBT_CACHE_KEY_VERSION 1.1.25 → 1.1.26

Disk cache, Linux signature verification, and the non-Windows paths are unchanged.

[eed3si9n]

Do you know where the bug is coming from? Is unzip somehow failing on Windows?

[eed3si9n]

I wonder if we can minimize this PR to this line only.

[eed3si9n]

I'm trying this here - #112