pmb.build.backend.run_abuild: remove cross default

This function doesn't handle the case of being called without a value
set for cross (it does not autodetect it) and callers don't expect that.
So unset the default value.

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2568

pmb/build/_package.py

@@ -714,9 +714,9 @@ def packages(
                 pkg["pkgver"],
                 channel,
                 pkg_arch,
+                cross,
                 strict,
                 force,
-                cross,
                 hostchroot,
                 src,
                 bootstrap_stage,

pmb/build/backend.py

@@ -190,9 +190,9 @@ def run_abuild(
     pkgver: str,
     channel: str,
     arch: Arch,
+    cross: CrossCompileType,
     strict: bool = False,
     force: bool = False,
-    cross: CrossCompileType = None,
     hostchroot: Chroot = Chroot.native(),
     src: str | None = None,
     bootstrap_stage: int = BootstrapStage.NONE,