Workaround Terraform v1.1 file provisioner regression

* Terraform v1.1 changed the behavior of provisioners and
`remote-exec` in a way that breaks support for expansions
in commands (including file provisioner, where `destination`
is part of an `scp` command)
* Terraform will likely revert the change eventually, but I
suspect it will take a while
* Instead, we can stop relying on Terraform's expansion
behavior. `/home/core` is a suitable choice for `$HOME` on
both Flatcar Linux and Fedora CoreOS (harldink `/var/home/core`)

Rel: hashicorp/terraform#30243

ssh.tf

@@ -24,7 +24,7 @@ resource "null_resource" "copy-controller-secrets" {
 
   provisioner "file" {
     content     = join("\n", local.assets_bundle)
-    destination = "$HOME/assets"
+    destination = "/home/core/assets"
   }
 
   provisioner "remote-exec" {