fix project_install_prefix for generate-github-actions

Summary:
X-link: facebook/sapling#952

fix getdeps generate-github-actions by adding missing process_project_dir_arguments.  Without this couldn't specify any project_install_prefix other that the default /usr/local

mononoke installs directly to getdeps inst dir so needs a project_install_prefix of / for the artifact discovery to work

regenerated the gh actions with `python3 fbcode/opensource/fbcode_builder/getdeps.py generate-github-actions mononoke --os-type=linux --allow-system-packages --output-dir fbcode/eden/oss/.github/workflows --job-file-prefix mononoke_ --job-name-prefix "Mononoke " --free-up-disk --project-install-prefix mononoke:/`

Resolves facebook/sapling#922

Reviewed By: bigfootjon

Differential Revision: D63031271

fbshipit-source-id: 768a4fbd7617c6061eca66b4d916ac25a4b66be9

Author: ahornby

build/fbcode_builder/getdeps.py

@@ -960,6 +960,7 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
         build_opts = setup_build_options(args, platform)
         ctx_gen = build_opts.get_context_generator()
         loader = ManifestLoader(build_opts, ctx_gen)
+        self.process_project_dir_arguments(args, loader)
         manifest = loader.load_manifest(args.project)
         manifest_ctx = loader.ctx_gen.get_context(manifest.name)
         run_on = self.get_run_on(args)
@@ -1147,8 +1148,10 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
 
             project_prefix = ""
             if not build_opts.is_windows():
-                project_prefix = (
-                    " --project-install-prefix %s:/usr/local" % manifest.name
+                prefix = loader.get_project_install_prefix(manifest) or "/usr/local"
+                project_prefix = " --project-install-prefix %s:%s" % (
+                    manifest.name,
+                    prefix,
                 )
 
             # If we have dep from same repo, we already built it and don't want to rebuild it again