Simplify logic for prefixing fastly spec to file

[Integralist]

Some users weren't seeing the fastly.toml spec URL in their configuration file (the logic was working for the majority of users who tested the code, but there appears to be an intermittent issue which would cause the spec URL insertion to fail).

An internal recommendation was made to simplify the logic (described below), and that's what has been done in this PR (which we expect to also resolve the issue reported internally with the spec URL being omitted).

The original flow was to encode data to disk and then seek the start of the file, checking if the spec URL was absent, and if so prepend it to the file. This complex flow was done because previously the toml dependency would truncate the entire file and remove the fastly spec URL, but since changing to a new (maintained) toml dependency that truncation no longer happens when encoding data to disk.

This means the flow for writing the config back to disk is now simply:

• Truncate fastly.toml.
• Write spec URL to disk.
• Encode the configuration data to disk (leaving the existing content unaffected).