[fontc] Only crater needs unique build file names

When running in the context of crater, gftools builder may be invoked
multiple times in parallel, which can cause the generated ninja.build
files to overwrite each other.

To fix this, I implemented a scheme where we generate a unique name for
the generated build file, so that multiple could exist and be in use for
the same project, at the same time.

These files can pile up, though; I had initially thought they were
cleaned up by default, but that doesn't seem to be the case.

This patch makes it so we only use this new naming scheme if we're
running for crater, and uses the old names otherwise.

It also makes it so we always set the cleanup flag if we're running for
crater.

Lib/gftools/builder/__init__.py

@@ -7,7 +7,6 @@
 from os import chdir
 from pathlib import Path
 from tempfile import NamedTemporaryFile, gettempdir
-import time
 from typing import Any, Dict, List, Union
 
 from gftools.builder.fontc import FontcArgs
@@ -64,7 +63,7 @@ def __init__(
         fontc_args.modify_config(self.config)
 
         self.known_operations = OperationRegistry(use_fontc=fontc_args.use_fontc)
-        self.ninja_file_name = f"build-{time.time_ns()}.ninja"
+        self.ninja_file_name = fontc_args.build_file_name()
         self.writer = Writer(open(self.ninja_file_name, "w"))
         self.named_files = {}
         self.used_operations = set([])

Lib/gftools/builder/fontc.py

@@ -10,6 +10,7 @@
 from argparse import Namespace
 from pathlib import Path
 from typing import Union
+import time
 
 from gftools.builder.file import File
 from gftools.builder.operations.fontc import set_global_fontc_path
@@ -35,6 +36,15 @@ def __init__(self, args: Union[Namespace, None]) -> None:
     def use_fontc(self) -> bool:
         return self.fontc_bin_path is not None
 
+    def build_file_name(self) -> str:
+        if self.fontc_bin_path or self.simple_output_path:
+            # if we're running for fontc we want uniquely named build files,
+            # to ensure they don't collide
+            return f"build-{time.time_ns()}.ninja"
+        else:
+            # otherwise just ues the default name
+            return "build.ninja"
+
     # update the config dictionary based on our special needs
     def modify_config(self, config: dict):
         if self.single_source:
@@ -52,6 +62,7 @@ def modify_config(self, config: dict):
             config["buildWebfont"] = False
             config["buildSmallCap"] = False
             config["splitItalic"] = False
+            config["cleanUp"] = True
             # disable running ttfautohint, because we had a segfault
             config["autohintTTF"] = False
             # set --no-production-names, because it's easier to debug