e2e: fix destination of templates in VaultSecrets test (#9146)

The `$NOMAD_SECRETS_DIR` environment variable is rendered as `/secrets`, which
prior to the recent security patch would unintentionally escape the file
sandbox and get dropped in a directory named `/secrets` where the Nomad client
binary was running. The `VaultSecrets` test was accidentally relying on this
behavior and that causes the test to fail.

e2e/vaultsecrets/input/secrets.nomad

@@ -33,7 +33,7 @@ job "secrets" {
 {{ end }}
 EOT
 
-        destination = "${NOMAD_SECRETS_DIR}/certificate.crt"
+        destination = "secrets/certificate.crt"
         change_mode = "noop"
       }
 
@@ -42,7 +42,7 @@ EOT
 SOME_SECRET={{ with secret "secrets-TESTID/data/myapp" }}{{- .Data.data.key -}}{{end}}
 EOT
 
-        destination = "${NOMAD_SECRETS_DIR}/access.key"
+        destination = "secrets/access.key"
       }
 
       resources {