build-aux: Disable file port name canonicalization in a few places.

I'm assuming this doesn't have any meaningful effect, and it seems to reduce
the number of readlink calls by a lot.

* build-aux/build-self.scm (build-program): Set
%file-port-name-canonicalization to #f.
(build): Ditto.
* build-aux/compile-as-derivation.scm: Ditto.

Change-Id: If43415c46c3911e84c76d1a9828b9c417a140a1b

build-aux/build-self.scm

@@ -111,6 +111,9 @@ build daemon, from within the generated build program."
                   #~(begin
                       (use-modules (ice-9 match))
 
+                      ;; To avoid lots of readlink calls
+                      (fluid-set! %file-port-name-canonicalization #f)
+
                       (eval-when (expand load eval)
                         ;; (gnu packages …) modules are going to be looked up
                         ;; under SOURCE.  (guix config) is looked up in FRONT.
@@ -246,6 +249,9 @@ Display a spinner when nothing happens."
                 #:rest rest)
   "Return a derivation that unpacks SOURCE into STORE and compiles Scheme
 files."
+  ;; Avoid lots of readlink calls
+  (fluid-set! %file-port-name-canonicalization #f)
+
   ;; Build the build program and then use it as a trampoline to build from
   ;; SOURCE.
   (mlet %store-monad ((build  (build-program source version guile-version

build-aux/compile-as-derivation.scm

@@ -20,6 +20,9 @@
 
 (use-modules (srfi srfi-26))
 
+;; Avoid lots of readlink calls
+(fluid-set! %file-port-name-canonicalization #f)
+
 ;; Add ~/.config/guix/current to the search path.
 (eval-when (expand load eval)
   (and=> (or (getenv "XDG_CONFIG_HOME")