fix(forwarder): improve variable checks #178

Job	Run time
permission_check	1s
prepare_matrix	20s
test-integration (./modules/forwarder)	2m 43s
test-integration (./modules/logwriter)	2m 21s
test-integration (./modules/config)	26s
test-integration (./modules/stack)	4m 28s
test-integration (./modules/metricstream)	1m 41s
test-integration (./modules/configsubscription)	56s
test-integration (./examples/stack-http)	2m 30s
test-integration (./examples/simple)	1m 39s
test-integration (./examples/forwarder-sns)	2m 28s
test-integration (./examples/logwriter-manual-subscription)	1m 35s
test-integration (./examples/logwriter-auto-subscription)	2m 39s
test-integration (./examples/forwarder-kms)	2m 20s
test-integration (./examples/stack-filedrop)	2m 25s
cleanup	10s
	28m 42s

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