Work around a bug in Chef's snap_package resource

https://github.com/chef/chef/issues/8827

`snap install` returns 0 when a package is already installed, so no need
for a guard clause

site-cookbooks/kosmos-bitcoin/recipes/snapd.rb

@@ -25,9 +25,9 @@
 # THE SOFTWARE.
 #
 
-package 'snapd'
-
-snap_package 'bitcoin-core'
+# FIXME: Switch to the snap_package resource when it gets fixed:
+# https://github.com/chef/chef/issues/8827
+execute "snap install bitcoin-core"
 
 bitcoin_user    = node['bitcoin']['username']
 bitcoin_group   = node['bitcoin']['usergroup']