Protect the `.Conn` property of `tabletHealthCheck` from being read/modified concurrently. #43

Job	Run time
Set push variable	1s
Build and push lite (latest)	2m 1s
Build and push lite (percona80)	1m 59s
Build and push vttestserver (mysql80)	1m 46s
Build and push (bullseye, vtadmin)	1m 11s
Build and push (bullseye, vtorc)	44s
Build and push (bullseye, vtgate)	39s
Build and push (bullseye, vttablet)	43s
Build and push (bullseye, mysqlctld)	34s
Build and push (bullseye, mysqlctl)	35s
Build and push (bullseye, vtctl)	36s
Build and push (bullseye, vtctlclient)	43s
Build and push (bullseye, vtctld)	1m 14s
Build and push (bullseye, vtctldclient)	39s
Build and push (bullseye, logrotate)	21s
Build and push (bullseye, logtail)	30s
Build and push (bullseye, vtbackup)	36s
Build and push (bullseye, vtexplain)	47s
Build and push (bookworm, vtadmin)	1m 5s
Build and push (bookworm, vtorc)	29s
Build and push (bookworm, vtgate)	26s
Build and push (bookworm, vttablet)	38s
Build and push (bookworm, mysqlctld)	30s
Build and push (bookworm, mysqlctl)	38s
Build and push (bookworm, vtctl)	59s
Build and push (bookworm, vtctlclient)	37s
Build and push (bookworm, vtctld)	1m 23s
Build and push (bookworm, vtctldclient)	28s
Build and push (bookworm, logrotate)	35s
Build and push (bookworm, logtail)	29s
Build and push (bookworm, vtbackup)	37s
Build and push (bookworm, vtexplain)	39s
	25m 12s

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