Use Fedora 35 for build environment

I needed to go back to Fedora 35 here because vagrant requires
an older version of Ruby than what is in Fedora 36.

```
Bundler found conflicting requirements for the Ruby version:
  In Gemfile:
    vagrant was resolved to 2.2.19, which depends on
      Ruby (>= 2.6, < 3.1)

  Current Ruby version:
    Ruby (= 3.1.2)
```

There is an issue upstream to track bumping the Vagrant maximum
Ruby version: hashicorp/vagrant#12638 (comment)

build.sh

@@ -1,7 +1,7 @@
 #!/bin/bash -x
 set -ex
 
-ctr=$(buildah from registry.fedoraproject.org/fedora:36)
+ctr=$(buildah from registry.fedoraproject.org/fedora:35)
 
 rpms=(
   make gcc ruby ruby-devel redhat-rpm-config # for building gems