chore: added E2E variable

Added variable so that the additional E2E test is also executed. This
test must be refactored in a future commit/ removed, as it depends on an
image already being present on the machine running the test.

Makefile

@@ -7,7 +7,7 @@ PORT := 5000
 .PHONY: test-e2e
 test-e2e:
 	@echo "Running e2e tests..."
-	@go test -v -race -count 1 ./test/
+	@export COSIGN_E2E="42" && go test -v -race -count 1 ./test/
 
 .PHONY: test-unit
 test-unit: