Fix building packages when git is in chroot

Building packages without git installed caused error messages from git
appearing on the screen, and I patched it upstream (see #1209).
However, this introduced a regression: when git *is* installed and the
aport folder *is not* part of a git repository (like we do it with
pmbootstrap), the build gets aborted now with git complaining that
the folder is not a valid git folder.

But the only case where this happens is, when abuild is trying to
include the git repositories' commit hash for the APKINDEX description.
This pmbootstrap commit hardcodes "postmarketOS" as APKINDEX
description, so it does not fail anymore.

pmb/build/_package.py

@@ -351,7 +351,7 @@ def run_abuild(args, apkbuild, arch, strict=False, force=False, cross=None,
         env["DISTCC_HOSTS"] = "127.0.0.1:" + args.port_distccd
 
     # Build the abuild command
-    cmd = ["abuild"]
+    cmd = ["abuild", "-D" "postmarketOS"]
     if strict:
         cmd += ["-r"]  # install depends with abuild
     else: