Have arg_parse perform the interpolations in rustc_action arguments

Summary:
I just noticed there is a built-in way in the Python `argparse` library to do this. We don't need to parse the command line as `List[str]` and then manually replace the parsed list with one that has a function applied to each argument.

We already use a similar `type=key_value_arg` for flags like `--env`, where it's responsible for splitting `--env K=V` into a tuple `("K", "V")`.

https://www.internalfb.com/code/fbsource/[a6d422d969cd7e6615d6992fbcc52a772ab16954]/fbcode/buck2/prelude/rust/tools/rustc_action.py?lines=79-85

https://www.internalfb.com/code/fbsource/[a6d422d969cd7e6615d6992fbcc52a772ab16954]/fbcode/buck2/prelude/rust/tools/rustc_action.py?lines=44-48

Reviewed By: zertosh

Differential Revision: D58790340

fbshipit-source-id: 4f92d96d00fa60501bc34f418f54479e82937e76

prelude/rust/tools/rustc_action.py

@@ -127,7 +127,7 @@ def arg_parse() -> Args:
     parser.add_argument(
         "rustc",
         nargs=argparse.REMAINDER,
-        type=str,
+        type=arg_eval,
         help="Compiler command line",
     )
 
@@ -235,7 +235,6 @@ async def handle_output(  # noqa: C901
 
 async def main() -> int:
     args = arg_parse()
-    args = args._replace(rustc=[arg_eval(arg) for arg in args.rustc])
 
     if args.echo:
         args.echo.write("".join(arg + "\n" for arg in args.rustc).encode("utf-8"))