Fail more clearly if the file program is not installed on the runner

[mbg]

This PR checks more clearly whether we can successfully find the location of the file program and emits a clearer message in the build log as well as a warning-level diagnostic if it cannot be found. There is also a new integration test which checks that this works as intended.

Since the check for whether the Go workaround is needed takes place before the database gets initialised, this PR also modifies the code for diagnostics to:

• Check if the database exists yet before writing a diagnostic.
• If it does not exist, the diagnostic is added to an array and flushed to disk later once the database is presumed to exist (init Action only)

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[henrymercer]

Very nice, I expect the functionality for writing diagnostics before the database has been initialized will come in handy elsewhere too.

You might want to consider adding a changelog note, otherwise LGTM!

[mbg]

Added a changelog entry