Only apply repl.ReplaceAll() on values from literal variable names
(e.g. map outputs), not on values resolved from placeholder keys
(e.g. {http.request.header.*}). The placeholder path already resolves
the value via repl.Get(), so a second expansion allows user-controlled
input containing {env.*} or {file.*} to be evaluated, leaking
environment variables and file contents.
Add regression test to verify placeholder-sourced values are not
re-expanded.
7e83775e3a