common/xbps-src/shutils/chroot.sh: support importing other keys

When packages in `hostdir/binpkgs` are signed, it causes `xbps-src` to
annoyingly request to import the associated key (every time, in the case
of automation tools like `xxbuild` or `ymir`). This allows the user to
add their personal public key `.plist` to `etc/repo-keys/` so this
prompting does not happen.

.gitignore

@@ -21,3 +21,4 @@ etc/conf
 etc/conf.*
 etc/virtual
 etc/xbps.d/custom
+etc/repo-keys