CI: mark external subdirs safe for FreeBSD build

```
fatal: detected dubious ownership in repository at '/home/runner/work/lightning/lightning/external/libwally-core'
  To add an exception for this directory, call:

  	git config --global --add safe.directory /home/runner/work/lightning/lightning/external/libwally-core
  Reinitializing submodules src/secp256k1 ...
  fatal: detected dubious ownership in repository at '/home/runner/work/lightning/lightning/external/libwally-core'
  To add an exception for this directory, call:

  	git config --global --add safe.directory /home/runner/work/lightning/lightning/external/libwally-core
  fatal: detected dubious ownership in repository at '/home/runner/work/lightning/lightning/external/libwally-core'
  To add an exception for this directory, call:

  	git config --global --add safe.directory /home/runner/work/lightning/lightning/external/libwally-core
  gmake: *** [external/Makefile:65: submodcheck] Error 128
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

.github/workflows/bsd.yml

@@ -59,6 +59,7 @@ jobs:
           git clone https://github.com/lightning/bolts.git ../bolts
           # fatal: unsafe repository ('/Users/runner/work/lightning/lightning' is owned by someone else)
           git config --global --add safe.directory `pwd`
+          for d in libsodium libwally-core gheap jsmn libbacktrace lowdown; do git config --global --add safe.directory `pwd`/external/$d; done
           git submodule update --init --recursive
 
           ./configure CC="$CC" --disable-valgrind