Fix how we cancel the context in the builtin backup engine (#17285) #458

Job	Run time
Build and push lite (latest)	4m 38s
Build and push lite (percona80)	4m 36s
Build and push vttestserver (mysql80)	2m 24s
Build and push (bullseye, vtadmin)	1m 26s
Build and push (bullseye, vtorc)	46s
Build and push (bullseye, vtgate)	44s
Build and push (bullseye, vttablet)	48s
Build and push (bullseye, mysqlctld)	52s
Build and push (bullseye, mysqlctl)	46s
Build and push (bullseye, vtctl)	49s
Build and push (bullseye, vtctlclient)	44s
Build and push (bullseye, vtctld)	1m 38s
Build and push (bullseye, vtctldclient)	44s
Build and push (bullseye, logrotate)	26s
Build and push (bullseye, logtail)	32s
Build and push (bullseye, vtbackup)	49s
Build and push (bullseye, vtexplain)	44s
Build and push (bookworm, vtadmin)	1m 28s
Build and push (bookworm, vtorc)	45s
Build and push (bookworm, vtgate)	47s
Build and push (bookworm, vttablet)	55s
Build and push (bookworm, mysqlctld)	56s
Build and push (bookworm, mysqlctl)	54s
Build and push (bookworm, vtctl)	47s
Build and push (bookworm, vtctlclient)	40s
Build and push (bookworm, vtctld)	1m 38s
Build and push (bookworm, vtctldclient)	46s
Build and push (bookworm, logrotate)	31s
Build and push (bookworm, logtail)	37s
Build and push (bookworm, vtbackup)	47s
Build and push (bookworm, vtexplain)	51s
Slack Notification if failed	0s
	35m 48s

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