Use an argsfile when calling merge_index_store.py

Summary:
## Problem

Previously, larger buck targets (those with several thousand transitive dependencies) would fail when building the `full-index-store`, because the CLI invocation would be too big and hit Linux limits (around [128KiB](https://unix.stackexchange.com/questions/110282/cp-max-source-files-number-arguments-for-copy-utility/)).

This would produce build failures like this:

```
$ buck build fbobjc/Apps/Instagram/Features/Sundial/IGSundial:IGSundial[full-index-store]
Action failed: fbsource//fbobjc/Apps/Instagram/Features/Sundial/IGSundial:IGSundial (merge_index_store __indexstore__/IGSundial/full_index_stores)
Local command returned non-zero exit code <no exit code>
Reproduce locally: `env -- 'BUCK_SCRATCH_PATH=buck-out/v2/tmp/fbsource/6e8c89e8087abbf6/merge_index_store/__indexstore__ ...<omitted>... ource/33ed8d67ac42318a/fbobjc/VendorLib/IGListKit/__IGListKit__/__indexstore__/IGListKit/index_store (run `buck2 log what-failed` to get the full command)`
```

See P1746393724 for an example output from `buck log what-failed`, it's clearly a ton of data.

## Solution

Instead of invoking `merge_index_store.py --source dir1 dir2 dir3 ...`, use an arguments file as `merge_index_store.py --source directories.txt` so the CLI invocation is short.

This is a pattern used in other buck rules, so I based this code on these examples:

https://www.internalfb.com/code/fbsource/[834112476033]/fbcode/buck2/prelude/cxx/symbols.bzl?lines=293

https://www.internalfb.com/code/fbsource/[fa0f657e098379489efb65a65b20ce0d269fb276][history]/fbcode/buck2/prelude/apple/swift/swift_compilation.bzl?lines=690

Differential Revision: D70747622

fbshipit-source-id: 4f7f187e3e01352ca2da687b46591dc8601335a8

prelude/apple/tools/index/merge_index_store.py

@@ -48,7 +48,15 @@ def merge_directories(source: str, destination: str) -> None:
 def main() -> None:
     args = parse_arguments()
     destination = args.dest
-    directories = args.sources
+
+    directories = []
+    for source in args.sources:
+        if source.startswith("@"):
+            with open(source[1:]) as f:
+                for line in f.readlines():
+                    directories.append(line.strip())
+        else:
+            directories.append(source)
 
     Path(destination).mkdir(parents=True, exist_ok=True)
 

prelude/cxx/index_store.bzl

@@ -58,11 +58,19 @@ def _merge_index_store(ctx: AnalysisContext, index_stores: list[Artifact] | Tran
 
     cmd = cmd_args([merge_index_store_tool])
     cmd.add(["--dest", merged_index_store.as_output()])
-    cmd.add(["--sources"])
+
+    txt_args = cmd_args()
     if isinstance(index_stores, list):
-        cmd.add(index_stores)
+        txt_args.add(index_stores)
     else:
-        cmd.add(index_stores.project_as_args("args"))
+        txt_args.add(index_stores.project_as_args("args"))
+
+    argsfile_path = merge_output_dir_name + ".files.txt"
+    argsfile = ctx.actions.write(argsfile_path, txt_args)
+    argsfile = cmd_args(argsfile, format = "@{}", hidden = txt_args)
+
+    cmd.add(["--sources"])
+    cmd.add(argsfile)
 
     # use prefer_remote = True here, it would have two following advantages
     # 1. Each bucket will perform a merge on RE,  which will fully utilize the high-speed network for materalizaion