If the LZMA archive is missing, the first-run message is printed every time.
This commit fixes that.
Split the first-run message into two pieces:
- The first-run (welcome and telemetry) message is printed only once, no matter
whether the cache was primed or not.
- The cache-priming message is printed only if the cache is avaialble.
Otherwise skip the cache introduction and the ccache priming operation.
