pmb.build._package.build_depends: update APKINDEX

When 'pmbootstrap build' is started with '--no-depends', we make sure
that binary packages of all dependencies exist before proceeding with a
build. Make sure that we have downloaded the APKINDEX for the given arch
first, otherwise this may complain that a binary package is missing
although it does exist.

This happened when using the "native" cross compile method, e.g. when
building kernels for different architectures. It would complain that
there is no binary package for "bash" (which is in Alpine obviously):
https://builds.sr.ht/~postmarketos/job/103882

pmb/build/_package.py

@@ -143,6 +143,7 @@ def build_depends(args, apkbuild, arch, strict):
     # --no-depends: check for binary packages
     depends_built = []
     if "no_depends" in args and args.no_depends:
+        pmb.helpers.repo.update(args, arch)
         for depend in depends:
             if not pmb.parse.apkindex.package(args, depend, arch, False):
                 raise RuntimeError("Missing binary package for dependency '" +