fix(tvix/eval): use coerce_to_string in builtins.substring

This actually uses coercion under the hood in C++ Nix. See the test
for an example.

Change-Id: Id56b364acf269225b6829d0b600e0222f8b3608d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8322
Reviewed-by: andi <andi@notmuch.email>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.exp

@@ -0,0 +1 @@
+"42"

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.nix

@@ -0,0 +1,5 @@
+# builtins.substring uses string coercion internally
+
+builtins.substring 0 2 {
+  __toString = _: "4200";
+}