helm: Use k8s `EnvVar` for `proxy.ExperimentalEnv` #3395

Job	Run time
cleanup	3s
changed-policy	13s
tag	4s
changed-multicluster	13s
changed-viz	9s
build-cli	2m 38s
build-ext (jaeger-webhook)	2m 20s
build-core (controller)	2m 42s
build-ext (metrics-api)	2m 12s
build-core (policy-controller)	5m 13s
build-ext (tap)	2m 16s
build-core (proxy)	2m 29s
build-ext (web)	4m 2s
test-policy	0s
test-viz	0s
test-ext (cluster-domain)	3m 9s
test-ext (default-policy-deny)	3m 27s
test-multicluster	0s
test-ext (external)	4m 34s
test-ext (rsa-ca)	2m 13s
test-core (cni-calico-deep)	3m 51s
test-ext (helm-upgrade)	10m 6s
test-core (deep)	2m 57s
test-ext (uninstall)	3m 29s
test-ext (upgrade-edge)	4m 32s
test-ext (upgrade-stable)	5m 5s
	1h 7m 57s

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