search-paths: Define the 'PATH' environment variable.

* guix/search-paths.scm ($PATH): New variable.
* guix/scripts/environment.scm (for-each-search-path): Use it.

guix/scripts/environment.scm

@@ -57,9 +57,7 @@ path value is appended."
                         (if (and current (not pure?))
                             (string-append value separator current)
                             value)))))
-              (cons* (search-path-specification
-                      (variable "PATH")
-                      (files '("bin" "sbin")))
+              (cons* $PATH
                      (delete-duplicates
                       (append-map package-native-search-paths inputs))))))
 

guix/search-paths.scm

@@ -31,6 +31,8 @@
             search-path-specification-file-type
             search-path-specification-file-pattern
 
+            $PATH
+
             search-path-specification->sexp
             sexp->search-path-specification
             evaluate-search-paths
@@ -58,6 +60,13 @@
   (file-pattern search-path-specification-file-pattern ;#f | string
                 (default #f)))
 
+(define $PATH
+  ;; The 'PATH' variable.  This variable is a bit special: it is not attached
+  ;; to any package in particular.
+  (search-path-specification
+   (variable "PATH")
+   (files '("bin" "sbin"))))
+
 (define (search-path-specification->sexp spec)
   "Return an sexp representing SPEC, a <search-path-specification>.  The sexp
 corresponds to the arguments expected by `set-path-environment-variable'."