Task: Implement Hotfix: fix the Linux sccache save

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(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

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The Stop step in continuous-linux.yml and nightly-linux.yml now waits for the sccache process to exit and for the cache directory to go quiet for three seconds before the save starts. The old step returned as soon as --stop-server returned, so tar read files that the server still wrote. A new step asks the service whether the key was stored. If it was not, the workflow waits again and saves once more. The old entry of the variant is still deleted before the save, because three variants do not fit in the 10 GB allowance. Only a CI run proves the fix. The hit rate of about 2% is a separate cause, and this change does not touch it.