[llvm] [lit] Expand late substitutions before running builtins (PR #165140)

[Aiden Grossman via llvm-commits]

================
@@ -0,0 +1,6 @@
+## Tests that readfile works with the env builtin.
+# RUN: echo -n "hello" > %t.1
+# RUN: env TEST=%{readfile:%t.1} python3 -c "import os; print(os.environ['TEST'])"
----------------
boomanaiden154 wrote:

Good point. It's not set by default, but set in the tests that need it. I've added it to keep this portable.

https://github.com/llvm/llvm-project/pull/165140