build: fix !pmb:crossdirect (MR 2291)

With the !pmb:crossdirect option, the crossdirect compilation method
gets disabled. This means there is no /native directory mounted inside
the foreign arch chroot, all binaries inside the foreign arch chroot are
supposed to run in QEMU. Handle this in the apk_wrapper.sh script that
we use to redirect abuild-apk to /native/usr/bin/abuild-apk.

Fix for (currently with master_staging_systemd branch):
  >>> upower: Analyzing dependencies...
  /usr/local/bin/abuild-apk: line 11: /native/usr/bin/abuild-apk: not found

pmb/build/init.py

@@ -67,6 +67,14 @@ def init(args, suffix="native"):
         with open(chroot + "/tmp/apk_wrapper.sh", "w") as handle:
             content = f"""
                 #!/bin/sh
+
+                # With !pmb:crossdirect, cross compilation is entriely done
+                # in QEMU, no /native dir gets mounted inside the foreign arch
+                # chroot.
+                if ! [ -d /native ]; then
+                    exec /usr/bin/abuild-apk "$@"
+                fi
+
                 export LD_PRELOAD_PATH=/native/usr/lib:/native/lib
                 args=""
                 for arg in "$@"; do