[shellcheck] fix SC2155 warning (#8490)

Declare and assign separately to avoid masking return values.

script/make-pretty

@@ -64,15 +64,25 @@
 
 set -euo pipefail
 
-readonly OT_BUILD_JOBS=$(getconf _NPROCESSORS_ONLN)
-readonly OT_EXCLUDE_DIRS=(third_party doc/site)
+OT_BUILD_JOBS=$(getconf _NPROCESSORS_ONLN)
+readonly OT_BUILD_JOBS
 
-readonly OT_CLANG_SOURCES=('*.c' '*.cc' '*.cpp' '*.h' '*.hpp')
-readonly OT_MARKDOWN_SOURCES=('*.md')
-readonly OT_PYTHON_SOURCES=('*.py')
+OT_EXCLUDE_DIRS=(third_party doc/site)
+readonly OT_EXCLUDE_DIRS
 
-readonly OT_CLANG_TIDY_FIX_DIRS=('examples' 'include' 'src' 'tests')
-readonly OT_CLANG_TIDY_BUILD_OPTS=(
+OT_CLANG_SOURCES=('*.c' '*.cc' '*.cpp' '*.h' '*.hpp')
+readonly OT_CLANG_SOURCES
+
+OT_MARKDOWN_SOURCES=('*.md')
+readonly OT_MARKDOWN_SOURCES
+
+OT_PYTHON_SOURCES=('*.py')
+readonly OT_PYTHON_SOURCES
+
+OT_CLANG_TIDY_FIX_DIRS=('examples' 'include' 'src' 'tests')
+readonly OT_CLANG_TIDY_FIX_DIRS
+
+OT_CLANG_TIDY_BUILD_OPTS=(
     '-DCMAKE_EXPORT_COMPILE_COMMANDS=ON'
     '-DOT_ANYCAST_LOCATOR=ON'
     '-DOT_APP_RCP=OFF'
@@ -129,8 +139,9 @@ readonly OT_CLANG_TIDY_BUILD_OPTS=(
     '-DOT_COMPILE_WARNING_AS_ERROR=ON'
     '-DOT_UPTIME=ON'
 )
+readonly OT_CLANG_TIDY_BUILD_OPTS
 
-readonly OT_CLANG_TIDY_CHECKS="\
+OT_CLANG_TIDY_CHECKS="\
 -*,\
 google-explicit-constructor,\
 google-readability-casting,\
@@ -149,8 +160,7 @@ readability-redundant-member-init,\
 readability-simplify-boolean-expr,\
 readability-static-accessed-through-instance,\
 "
-
-#performance-for-range-copy\
+readonly OT_CLANG_TIDY_CHECKS
 
 do_clang_format()
 {