pmb.build: make sure cross compiler config is appropriate for each given package

Here's the problem: Imagine a queue with 2 packages in it, the 1st
package uses crossdirect and the second uses cross-native. When building
the 1st package, pmb will configure the native chroot for crossdirect
as expected. When it gets to the 2nd package, the chroot env/config
might not be appropriate for actually doing a native cross compile.

This re-inits the cross compiler stuff if the cross compile method
changes while processing the queue. Another approach that might solve
this problem is to not re-use chroots when building packages...

pmb/build/_package.py

@@ -607,10 +607,10 @@ def packages(
             pkg_depends.append("rsync")
 
         # We only need to init cross compiler stuff once
-        if not cross:
-            cross = pmb.build.autodetect.crosscompile(pkg["apkbuild"], pkg_arch)
-            if cross:
-                pmb.build.init_compiler(context, pkg_depends, cross, pkg_arch)
+        prev_cross = cross
+        cross = pmb.build.autodetect.crosscompile(pkg["apkbuild"], pkg_arch)
+        if cross != prev_cross:
+            pmb.build.init_compiler(context, pkg_depends, cross, pkg_arch)
             if cross == "crossdirect":
                 pmb.chroot.mount_native_into_foreign(chroot)