Migrate minimal-xcode to Pika

Summary:
Flip the buck2 binary build from minimal-xcode to Pika via PACKAGE file modifier. Although there's a good amount of rust toolchain usage, there are C++ dependencies (e.g. remote execution client libs) that will be flipped to Pika.

Also, to ensure .td jobs pass (specifically one that tests modefile-less usage), made
```
ovr_config//build_mode/default_opt_cxx:enabled ->
[
  ovr_config//toolchain/fb/constraints:macos-minimal
  ovr_config//toolchain/xcode/force_minimal_xcode:yes
]
```
into
```
ovr_config//build_mode/default_opt_cxx:enabled ->
[
  ovr_config//toolchain/fb/constraints:macos-minimal
  ovr_config//toolchain/xcode/force_minimal_xcode:no
  ovr_config//toolchain/xcode/force_minimal_xcode:pika-fat-override
]
```

This allows an modefileless invocation like `buck2 build fbcode//buck2:buck2 -m x86_64`

Reviewed By: milend

Differential Revision: D65916701

fbshipit-source-id: c45a7cb4ac4098aae583902c8aaa6062145343e5

app/modifier.bzl

@@ -98,21 +98,6 @@ def buck2_modifiers():
                 "ovr_config//os:macos": "ovr_config//build_mode/constraints:fbcode-custom-allocators-enabled",
             }),
         }),
-        modifiers.conditional({
-            "DEFAULT": None,
-            "ovr_config//build_mode/default_opt_cxx:enabled": modifiers.conditional({
-                "DEFAULT": None,
-                "ovr_config//os:macos": "ovr_config//toolchain/fb/constraints:macos-minimal",
-            }),
-        }),
-        # TODO(scottcao): This modifier can be deleted if D61497000 lands successfully
-        modifiers.conditional({
-            "DEFAULT": None,
-            "ovr_config//build_mode/default_opt_cxx:enabled": modifiers.conditional({
-                "DEFAULT": None,
-                "ovr_config//os:macos": "ovr_config//toolchain/xcode/force_minimal_xcode:yes",
-            }),
-        }),
         modifiers.conditional({
             "DEFAULT": None,
             "ovr_config//build_mode/default_opt_cxx:enabled": modifiers.conditional({