Adding type signatures to `#audit_formula` methods in formula cops
would lead to verbose, repetitive signatures across the existing ~63
instances. This reworks `#audit_formula` to use a `T::Struct` for its
arguments, which allows us to use a one-line signature for these
methods.
When the file isn't world-readable, `brew audit` prints a failure
message including a suggestion to `chmod +r` the file. Unfortunately,
this isn't quite right: with both macOS and coreutils, leaving out the
"who" in a chmod only affects bits which would be set in the umask. So,
if the umask doesn't allow world-readable (which might be why the file
wasn't world-readable in the first place), the suggested chmod command
does nothing.
Change to print `chmod a+r` instead; that does have the intended effect.
No other `chmod` suggestions in this file have the same problem.
