cmd/cigocacher,go.mod: add logging for canceled PUTs (#20584)

Pulls in bradfitz/go-tool-cache#43 and:

* Add logging for canceled PUTs to ensure we have some visibility.
* Scale PUT timeouts with object size.
* Control the Shutdown timeout separately from the PUT timeout.

Updates tailscale/corp#45334

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>