build-self: Execute trampoline in a clean environment.

Previously execution of the trampoline would be somewhat sensitive to
GUILE_LOAD_PATH & co., for example.

* build-aux/build-self.scm (build-program): Remove 'unsetenv' call and
%LOAD-COMPILED-PATH hack.
(call-with-clean-environment): New procedure.
(with-clean-environment): New macro.
(build): Wrap 'open-pipe*' call in 'with-clean-environment'.

build-aux/build-self.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2016, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -293,9 +293,6 @@ (define fake-git
                       (use-modules (ice-9 match))
 
                       (eval-when (expand load eval)
-                        ;; Don't augment '%load-path'.
-                        (unsetenv "GUIX_PACKAGE_PATH")
-
                         ;; (gnu packages …) modules are going to be looked up
                         ;; under SOURCE.  (guix config) is looked up in FRONT.
                         (match (command-line)
@@ -312,15 +309,11 @@ (define fake-git
 
                         ;; Only load Guile-Gcrypt, our own modules, or those
                         ;; of Guile.
-                        (match %load-compiled-path
-                          ((front _ ... sys1 sys2)
-                           (unless (string-prefix? #$guile-gcrypt front)
-                             (set! %load-compiled-path
-                               (list (string-append #$guile-gcrypt
-                                                    "/lib/guile/"
-                                                    (effective-version)
-                                                    "/site-ccache")
-                                     front sys1 sys2))))))
+                        (set! %load-compiled-path
+                          (cons (string-append #$guile-gcrypt "/lib/guile/"
+                                               (effective-version)
+                                               "/site-ccache")
+                                %load-compiled-path)))
 
                       (use-modules (guix store)
                                    (guix self)
@@ -372,6 +365,19 @@ (define spin
                              derivation-file-name))))))
                   #:module-path (list source))))
 
+(define (call-with-clean-environment thunk)
+  (let ((env (environ)))
+    (dynamic-wind
+      (lambda ()
+        (environ '()))
+      thunk
+      (lambda ()
+        (environ env)))))
+
+(define-syntax-rule (with-clean-environment exp ...)
+  "Evaluate EXP in a context where zero environment variables are defined."
+  (call-with-clean-environment (lambda () exp ...)))
+
 ;; The procedure below is our return value.
 (define* (build source
                 #:key verbose? (version (date-version-string)) system
@@ -406,14 +412,17 @@ (define* (build source
       ;; stdin will actually be /dev/null.
       (let* ((pipe   (with-input-from-port port
                        (lambda ()
-                         (setenv "GUILE_WARN_DEPRECATED" "no") ;be quiet and drive
-                         (open-pipe* OPEN_READ
-                                     (derivation->output-path build)
-                                     source system version
-                                     (if (file-port? port)
-                                         (number->string
-                                          (logior major minor))
-                                         "none")))))
+                         ;; Make sure BUILD is not influenced by
+                         ;; $GUILE_LOAD_PATH & co.
+                         (with-clean-environment
+                          (setenv "GUILE_WARN_DEPRECATED" "no") ;be quiet and drive
+                          (open-pipe* OPEN_READ
+                                      (derivation->output-path build)
+                                      source system version
+                                      (if (file-port? port)
+                                          (number->string
+                                           (logior major minor))
+                                          "none"))))))
              (str    (get-string-all pipe))
              (status (close-pipe pipe)))
         (match str