feat(lib): backport with-environment-variables from 28.1

And alias letenv! to it. Not sure if I want to deprecate it though,
given the length of the macro it's linked to...

lisp/doom-lib.el

@@ -198,13 +198,8 @@ TRIGGER-HOOK is a list of quoted hooks and/or sharp-quoted functions."
         (buffer-file-name)
         ((error "Cannot get this file-path"))))
 
-(defmacro letenv! (envvars &rest body)
-  "Lexically bind ENVVARS in BODY, like `let' but for `process-environment'."
-  (declare (indent 1))
-  `(let ((process-environment (copy-sequence process-environment)))
-     ,@(cl-loop for (var val) in envvars
-                collect `(setenv ,var ,val))
-     ,@body))
+;; REVIEW Should I deprecate this? The macro's name is so long...
+(defalias 'letenv! 'with-environment-variables)
 
 (defmacro letf! (bindings &rest body)
   "Temporarily rebind function, macros, and advice in BODY.
@@ -827,5 +822,23 @@ inserted before contatenating."
                 collect (match-string 1 str)))
      "/")))
 
+;; Introduced in 28.1
+(unless (fboundp 'with-environment-variables)
+  (defmacro with-environment-variables (variables &rest body)
+    "Set VARIABLES in the environment and execute BODY.
+VARIABLES is a list of variable settings of the form (VAR VALUE),
+where VAR is the name of the variable (a string) and VALUE
+is its value (also a string).
+
+The previous values will be be restored upon exit."
+    (declare (indent 1) (debug (sexp body)))
+    (unless (consp variables)
+      (error "Invalid VARIABLES: %s" variables))
+    `(let ((process-environment (copy-sequence process-environment)))
+       ,@(mapcar (lambda (elem)
+                   `(setenv ,(car elem) ,(cadr elem)))
+                 variables)
+       ,@body)))
+
 (provide 'doom-lib)
 ;;; doom-lib.el ends here