fix(tvix/eval): don't thunk home relative paths

C++ Nix resolves home relative paths at [parse] time. This is not an
option for us, since it prevents being able to separate the compilation
and execution phase later (e.g. precompiled nix expressions). However, a
practical consequence of this is that paths expressions are always
literals (strict) and never thunks.

[parse]: 7066d21a0d/src/libexpr/parser.y (L518-L527)

Change-Id: Ie4b9dc68f62c86d6c7fd5f1c9460c850d97ed1ca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7041
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/tests/nix_oracle.rs

@@ -154,6 +154,9 @@ compare_lazy_eval_tests! {
     thunked_lambda_in_list("[ (x: x) ]");
     thunked_function_application_in_list("[ (builtins.add 1 2) ]");
     thunked_legacy_let_in_list("[ (let { foo = 12; body = foo; }) ]");
+    unthunked_relative_path("[ ./foo ]");
+    unthunked_home_relative_path("[ ~/foo ]");
+    unthunked_absolute_path("[ /foo ]");
 
     unthunked_formals_fallback_literal("({ foo ? 12 }: [ foo ]) { }");
     unthunked_formals_fallback_string_literal("({ foo ? \"wiggly\" }: [ foo ]) { }");