jenkins,win: move clcache cache out of user profile - #4472

Fixes #4471

compile.cmd sets CLCACHE_BASEDIR but not CLCACHE_DIR, so clcache stores its cache in %USERPROFILE%\clcache. That is the configuration reported in frerich/clcache#342 to fail with:

Error: [WinError 5] Access is denied: '...\clcache\stats.txt.new' -> '...\clcache\stats.txt'

and setting CLCACHE_DIR to a path outside the profile is the workaround confirmed by several people in that thread. Upstream is archived, so this has to be handled on our side.

Notes:

• The path is arbitrary — anything outside the user profile should do. C:\clcache_dir just matches the value used upstream. Happy to rename it; it is a little close to C:\clcache, which holds the installation.
• clcache creates the directory itself, but if the Jenkins account can't create it at the drive root, the ansible role could pre-create it instead.
• This abandons the existing cache, so the first builds on each machine will be cold. I think that's an acceptable trade against the current failure rate, but say the word if you'd rather migrate the directory.
• Untested — I have no access to the Windows workers or to CI console logs. Please treat CI as the source of truth.
• Assisted by Claude Code for the investigation; I checked the diff and the upstream issue myself.