apt-get update on github (#113)

Summary:
Pull Request resolved: facebook/sapling#113

GitHub Actions was failing at apt-get stage,  error message suggested adding an apt-get update as remedy.  Added in this diff and it indeed works.

The actions build fails later on missing mio::net module when building mysql async, but that's unrelated to this change.

Reviewed By: farnz

Differential Revision: D34368662

fbshipit-source-id: f0a00da3ee740ae4443a328616e792ea615c922c

Author: ahornby

build/fbcode_builder/getdeps.py

@@ -988,9 +988,13 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
                 build_opts.allow_system_packages
                 and build_opts.host_type.get_package_manager()
             ):
+                sudo_arg = "sudo "
                 allow_sys_arg = " --allow-system-packages"
+                if build_opts.host_type.get_package_manager() == "deb":
+                    out.write("    - name: Update system package info\n")
+                    out.write(f"      run: {sudo_arg}apt-get update\n")
+
                 out.write("    - name: Install system deps\n")
-                sudo_arg = "sudo "
                 if build_opts.is_darwin():
                     # brew is installed as regular user
                     sudo_arg = ""