require_bootstrap: ensure apkindex exists (MR 2287)

When the user attempts to build a package with systemd enabled,
pmbootstrap checks first if the postmarketos-base-systemd package is in
any of the APKINDEX files. Fetch the APKINDEXes before trying to find
the package in them.

I'm running into this currently with bpo, when attempting to build any
package from the master_staging_systemd branch.

pmb/helpers/repo_bootstrap.py

@@ -4,6 +4,7 @@ import logging
 import glob
 
 import pmb.config.pmaports
+import pmb.helpers.repo
 
 
 progress_done = 0
@@ -179,6 +180,7 @@ def require_bootstrap(args, arch, trigger_str):
     :param trigger_str: message for the user to understand what caused this
     """
     if pmb.config.other.is_systemd_selected(args):
+        pmb.helpers.repo.update(args, arch)
         pkg = pmb.parse.apkindex.package(args, "postmarketos-base-systemd",
                                          arch, False)
         if not pkg: