fix: calling of head and grep in DEBUG mode

In DEBUG mode, `head` and `grep` was incorrectly called with `''`.

This caused warnings like:
`grep: ^\s*((#|!)|(#\s*!)|(!\s*#))\s*(/usr(/local)?)?/bin/(env\s+)?(sh|ash|bash|dash|ksh|bats)\b: No such file or directory`

In the end, Differential ShellCheck, in some cases, couldn't identify
shell scripts correctly in DEBUG mode.

src/functions.sh

@@ -131,29 +131,23 @@ has_shebang () {
   [[ $# -le 0 ]] && return 1
   local file="$1"
 
-  ! is_debug && local quiet=--quiet
-
-  local grep_args=(
-    "${quiet:-}"
-  )
-
   # shell shebangs detection
-  if head -n1 "${file}" | grep "${grep_args[@]}" -E '^\s*((#|!)|(#\s*!)|(!\s*#))\s*(/usr(/local)?)?/bin/(env\s+)?(sh|ash|bash|dash|ksh|bats)\b'; then
+  if head -n1 "${file}" | grep ${RUNNER_DEBUG:+"--quiet"} -E '^\s*((#|!)|(#\s*!)|(!\s*#))\s*(/usr(/local)?)?/bin/(env\s+)?(sh|ash|bash|dash|ksh|bats)\b'; then
     return 0
   fi
 
   # ShellCheck shell directive detection
-  if grep "${grep_args[@]}" -E '^\s*#\s*shellcheck\s+shell=(sh|ash|bash|dash|ksh|bats)\s*' "${file}"; then
+  if grep  ${RUNNER_DEBUG:+"--quiet"} -E '^\s*#\s*shellcheck\s+shell=(sh|ash|bash|dash|ksh|bats)\s*' "${file}"; then
     return 0
   fi
 
   # Emacs mode detection
-  if grep "${grep_args[@]}" -E '^\s*#\s+-\*-\s+(sh|ash|bash|dash|ksh|bats)\s+-\*-\s*' "${file}"; then
+  if grep  ${RUNNER_DEBUG:+"--quiet"} -E '^\s*#\s+-\*-\s+(sh|ash|bash|dash|ksh|bats)\s+-\*-\s*' "${file}"; then
     return 0
   fi
 
   # Vi and Vim modeline filetype detection
-  if grep "${grep_args[@]}" -E '^\s*#\s+vim?:\s+(set\s+)?(ft|filetype)=(sh|ash|bash|dash|ksh|bats)\s*' "${file}"; then
+  if grep  ${RUNNER_DEBUG:+"--quiet"} -E '^\s*#\s+vim?:\s+(set\s+)?(ft|filetype)=(sh|ash|bash|dash|ksh|bats)\s*' "${file}"; then
     return 0
   fi