Merge bitcoin/bitcoin#24240: depends: fix capnp's descriptor for make download

01e121d depends: fix capnp's descriptor for make download (Cory Fields)

Pull request description:

  The non-native capnp was trying to fetch the wrong file.
  Without this, "make -C depends MULTIPROCESS=1 download" is broken.

  Presumably it breaks with the download target because the dependency graph is flattened. It manages to work if native_capnp is encountered first because it will then be found in the cache.

ACKs for top commit:
  gruve-p:
    tACK bitcoin/bitcoin@01e121d
  hebasto:
    ACK 01e121d, tested on Linux Mint 20.2 (x86_64).

Tree-SHA512: 2605d895f3799be5a311f6f7d36a5c13cdb715dc148915ad818f4afc7d5de92cd6b8ecd34ff2b21cef6743b090819bba1e3353096cfb5659c55f76113ce5adf3

Author: fanquake

depends/packages/capnp.mk

@@ -1,6 +1,7 @@
 package=capnp
 $(package)_version=$(native_$(package)_version)
 $(package)_download_path=$(native_$(package)_download_path)
+$(package)_download_file=$(native_$(package)_download_file)
 $(package)_file_name=$(native_$(package)_file_name)
 $(package)_sha256_hash=$(native_$(package)_sha256_hash)
 $(package)_dependencies=native_$(package)