Use &l:equalprg so local options do not override global ones

autoload/ale/fix/registry.vim

@@ -243,7 +243,7 @@ endfunction
 " (name, func, filetypes, desc, aliases)
 function! ale#fix#registry#Add(name, func, filetypes, desc, ...) abort
     " This command will throw from the sandbox.
-    let &equalprg=&equalprg
+    let &l:equalprg=&l:equalprg
 
     if type(a:name) isnot v:t_string
         throw '''name'' must be a String'

autoload/ale/linter.vim

@@ -53,7 +53,7 @@ endfunction
 " Do not call this function.
 function! ale#linter#GetLintersLoaded() abort
     " This command will throw from the sandbox.
-    let &equalprg=&equalprg
+    let &l:equalprg=&l:equalprg
 
     return s:linters
 endfunction
@@ -295,7 +295,7 @@ endfunction
 
 function! ale#linter#Define(filetype, linter) abort
     " This command will throw from the sandbox.
-    let &equalprg=&equalprg
+    let &l:equalprg=&l:equalprg
 
     if !has_key(s:linters, a:filetype)
         let s:linters[a:filetype] = []

autoload/ale/util.vim

@@ -268,7 +268,7 @@ endfunction
 " See :help sandbox
 function! ale#util#InSandbox() abort
     try
-        let &equalprg=&equalprg
+        let &l:equalprg=&l:equalprg
     catch /E48/
         " E48 is the sandbox error.
         return 1