fix: json encoding on bind variables #702

Job	Run time
Set push variable	1s
Build and push vttestserver (mysql80)	1m 48s
Build and push lite (latest)	2m 8s
Build and push lite (percona80)	2m 13s
Build and push (bullseye, vtadmin)	1m 11s
Build and push (bullseye, vtorc)	33s
Build and push (bullseye, vtgate)	39s
Build and push (bullseye, vttablet)	39s
Build and push (bullseye, mysqlctld)	34s
Build and push (bullseye, mysqlctl)	38s
Build and push (bullseye, vtctl)	25s
Build and push (bullseye, vtctlclient)	38s
Build and push (bullseye, vtctld)	1m 26s
Build and push (bullseye, vtctldclient)	33s
Build and push (bullseye, logrotate)	14s
Build and push (bullseye, logtail)	24s
Build and push (bullseye, vtbackup)	37s
Build and push (bullseye, vtexplain)	39s
Build and push (bookworm, vtadmin)	1m 10s
Build and push (bookworm, vtorc)	38s
Build and push (bookworm, vtgate)	29s
Build and push (bookworm, vttablet)	38s
Build and push (bookworm, mysqlctld)	43s
Build and push (bookworm, mysqlctl)	38s
Build and push (bookworm, vtctl)	37s
Build and push (bookworm, vtctlclient)	32s
Build and push (bookworm, vtctld)	1m 17s
Build and push (bookworm, vtctldclient)	27s
Build and push (bookworm, logrotate)	16s
Build and push (bookworm, logtail)	21s
Build and push (bookworm, vtbackup)	39s
Build and push (bookworm, vtexplain)	27s
	24m 12s

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