feat(gemfile): update with fix for recent rubocop violation

```
Gemfile:3:8: C: [Correctable] Style/FetchEnvVar:
Use ENV.fetch('PROXY_RUBYGEMSORG', 'https://rubygems.org') instead of
    ENV['PROXY_RUBYGEMSORG'] || 'https://rubygems.org'.
    (https://rubystyle.guide/#hash-fetch-defaults)
source ENV['PROXY_RUBYGEMSORG'] || 'https://rubygems.org'
       ^^^^^^^^^^^^^^^^^^^^^^^^
```

Gemfile

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-source ENV['PROXY_RUBYGEMSORG'] || 'https://rubygems.org'
+source ENV.fetch('PROXY_RUBYGEMSORG', 'https://rubygems.org')
 
 # Install the `inspec` gem using `git` because versions after `4.22.22`
 # suppress diff output; this version fixes this for our uses.

ssf/files/default/Gemfile

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-source ENV['PROXY_RUBYGEMSORG'] || 'https://rubygems.org'
+source ENV.fetch('PROXY_RUBYGEMSORG', 'https://rubygems.org')
 
 # Install the `inspec` gem using `git` because versions after `4.22.22`
 # suppress diff output; this version fixes this for our uses.