roachtest: pass env vars to roachprod via multiple --env flags

Roachprod now shell escapes environment variables when rendering its
start script. As a result, multiple environment variables passed as a
single string, such as:

    --env "COCKROACH_MEMPROF_INTERVAL=1m COCKROACH_DISABLE_QUIESCENCE=true"

would result in us exporting it as a single entry:

    export 'COCKROACH_MEMPROF_INTERVAL=1m COCKROACH_DISABLE_QUIESCENCE=true'

which results in a single environment variable getting exported

    COCKROACH_MEMPROF_INTERVAL='1m COCKROACH_DISABLE_QUIESCENCE=true'

I've converted the tests that were passing multiple environment
variables in this was to passing them via multiple invocations of the
`--env` flag.

This seem preferable than teaching roachprod more sophisticated
environment variable parsing.

Release note: None

pkg/cmd/roachtest/tests/clearrange.go

@@ -79,7 +79,8 @@ func runClearRange(ctx context.Context, t test.Test, c cluster.Cluster, aggressi
 		//
 		// NB: the below invocation was found to actually make it to the server at the time of writing.
 		opts = append(opts, option.StartArgs(
-			"--env", "COCKROACH_CONSISTENCY_AGGRESSIVE=true COCKROACH_ENFORCE_CONSISTENT_STATS=true",
+			"--env", "COCKROACH_CONSISTENCY_AGGRESSIVE=true",
+			"--env", "COCKROACH_ENFORCE_CONSISTENT_STATS=true",
 		))
 	}
 	c.Start(ctx, opts...)

pkg/cmd/roachtest/tests/kv.go

@@ -628,9 +628,8 @@ func registerKVSplits(r registry.Registry) {
 				c.Put(ctx, t.Cockroach(), "./cockroach", c.Range(1, nodes))
 				c.Put(ctx, t.DeprecatedWorkload(), "./workload", c.Node(nodes+1))
 				c.Start(ctx, c.Range(1, nodes), option.StartArgs(
-					// NB: this works. Don't change it or only one of the two vars may actually
-					// make it to the server.
-					"--env", "COCKROACH_MEMPROF_INTERVAL=1m COCKROACH_DISABLE_QUIESCENCE="+strconv.FormatBool(!item.quiesce),
+					"--env", "COCKROACH_MEMPROF_INTERVAL=1m",
+					"--env", "COCKROACH_DISABLE_QUIESCENCE="+strconv.FormatBool(!item.quiesce),
 					"--args=--cache=256MiB",
 				))