Fix style issues

[samford]

While fixing brew style tap support (Homebrew/brew#17482), brew CI surfaced some style issues in this tap. This fixes style issues but there are still a number of shellcheck issues in handler.sh that need to be handled in some way (either through fixes or disabling unwanted rules):

In /opt/homebrew/Library/Taps/homebrew/homebrew-command-not-found/handler.sh line 12:
if ! which brew >/dev/null; then return; fi
     ^---^ SC2230 (info): 'which' is non-standard. Use builtin 'command -v' instead.


In /opt/homebrew/Library/Taps/homebrew/homebrew-command-not-found/handler.sh line 22:
  if test -z "${CONTINUOUS_INTEGRATION}" && test -n "${MC_SID}" -o ! -t 1
              ^-----------------------^ SC2154 (warning): CONTINUOUS_INTEGRATION is referenced but not assigned.
                                                     ^-------^ SC2154 (warning): MC_SID is referenced but not assigned.


In /opt/homebrew/Library/Taps/homebrew/homebrew-command-not-found/handler.sh line 27:
    [[ -n "${ZSH_VERSION}" ]] && [[ "${ZSH_VERSION}" > "5.2" ]] &&
                                                       ^---^ SC2072 (error): Decimals are not supported. Either use integers only, or use bc or awk to compare.


In /opt/homebrew/Library/Taps/homebrew/homebrew-command-not-found/handler.sh line 34:
    local txt="$(brew which-formula --explain "${cmd}" 2>/dev/null)"
          ^-^ SC2155 (warning): Declare and assign separately to avoid masking return values.


In /opt/homebrew/Library/Taps/homebrew/homebrew-command-not-found/handler.sh line 43:
    [[ -n "${ZSH_VERSION}" ]] && [[ "${ZSH_VERSION}" > "5.2" ]] &&
                                                       ^---^ SC2072 (error): Decimals are not supported. Either use integers only, or use bc or awk to compare.

For more information:
  https://www.shellcheck.net/wiki/SC2072 -- Decimals are not supported. Eithe...
  https://www.shellcheck.net/wiki/SC2154 -- CONTINUOUS_INTEGRATION is referen...
  https://www.shellcheck.net/wiki/SC2155 -- Declare and assign separately to ...

[MikeMcQuaid]

Thanks again @samford!