fbcode_builder: keep going after build errors to surface all failures

Summary:
When CMake's `--build` runs through ninja, the build stops on the first error by default. In CI that means a single broken target masks every other failure in the build, and each red run only reveals one problem at a time.

Pass `-- -k 0` to ninja so it keeps going after errors. `-k 0` means "do not stop, regardless of failure count". The exit code of the build is still nonzero when any target fails, so this is purely an information-surfacing change — successful runs are unaffected.

getdeps always configures CMake with `-G Ninja` (see `_compute_cmake_define_args` and `CMakeBootstrap`), so the underlying build tool is always ninja. ninja requires `-k` to take a value and has no `--keep-going` long form (see `kLongOptions` in `ninja.cc`).

Reviewed By: vitaut

Differential Revision: D101570020

fbshipit-source-id: 912ad27b2eb790be0f64f074f9257c71f1f5f5ae

Author: r-barnes

build/fbcode_builder/getdeps/builder.py

@@ -1008,6 +1008,12 @@ def _build(self, reconfigure: bool) -> None:
                 self.build_opts.build_type,
                 "-j",
                 str(self.num_jobs),
+                "--",
+                # Continue building even if we see an error
+                # Increases dev velocity by surfacing all errors
+                # `-k 0` is understood by ninja - the only build system we use
+                "-k",
+                "0",
             ],
             env=env,
             preexec_fn=self.memory_limit_preexec_fn,
@@ -1047,6 +1053,12 @@ def _build_targets(self, targets: Sequence[str]) -> None:
                 self.build_opts.build_type,
                 "-j",
                 str(self.num_jobs),
+                "--",
+                # Continue building even if we see an error
+                # Increases dev velocity by surfacing all errors
+                # `-k 0` is understood by ninja - the only build system we use
+                "-k",
+                "0",
             ]
         )