fix(env): resolve nested dotenv template expansion deterministically

[SergioChan]

Summary

• resolve dotenv values with iterative template expansion so nested {{.VAR}} chains are evaluated consistently
• keep dotenv precedence behavior unchanged (first file wins), while making resolution order deterministic
• add a regression test for nested dotenv template chains (MID_DIR -> FINAL_DIR -> FULL_PATH)

Testing

• Not run locally: this environment has Go 1.22.2 and cannot download the required Go 1.25 toolchain (go.mod requires go >= 1.25)
• Added targeted regression test: TestResolveDotenvVarsNestedTemplates in variables_dotenv_test.go

Related

Fixes #1847