guix build: Add '--repair'.

* guix/scripts/build.scm (show-help, %options): Add '--repair'.
* doc/guix.texi (Invoking guix gc): Mention 'guix build --repair'.
(Additional Build Options): Document it.

doc/guix.texi

@@ -2274,11 +2274,14 @@ traverses @emph{all the files in the store}, this command can take a
 long time, especially on systems with a slow disk drive.
 
 @cindex repairing the store
+@cindex corruption, recovering from
 Using @option{--verify=repair} or @option{--verify=contents,repair}
 causes the daemon to try to repair corrupt store items by fetching
 substitutes for them (@pxref{Substitutes}).  Because repairing is not
 atomic, and thus potentially dangerous, it is available only to the
-system administrator.
+system administrator.  A lightweight alternative, when you know exactly
+which items in the store are corrupt, is @command{guix build --repair}
+(@pxref{Invoking guix build}).
 
 @item --optimize
 @cindex deduplication
@@ -4859,6 +4862,14 @@ When used in conjunction with @option{--keep-failed}, the differing
 output is kept in the store, under @file{/gnu/store/@dots{}-check}.
 This makes it easy to look for differences between the two results.
 
+@item --repair
+@cindex repairing store items
+@cindex corruption, recovering from
+Attempt to repair the specified store items, if they are corrupt, by
+re-downloading or rebuilding them.
+
+This operation is not atomic and thus restricted to @code{root}.
+
 @item --derivations
 @itemx -d
 Return the derivation paths, not the output paths, of the given

guix/scripts/build.scm

@@ -485,6 +485,8 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
   -d, --derivations      return the derivation paths of the given packages"))
   (display (_ "
       --check            rebuild items to check for non-determinism issues"))
+  (display (_ "
+      --repair           repair the specified items"))
   (display (_ "
   -r, --root=FILE        make FILE a symlink to the result, and register it
                          as a garbage collector root"))
@@ -535,6 +537,12 @@ must be one of 'package', 'all', or 'transitive'~%")
                           (alist-cons 'build-mode (build-mode check)
                                       result)
                           rest)))
+         (option '("repair") #f #f
+                 (lambda (opt name arg result . rest)
+                   (apply values
+                          (alist-cons 'build-mode (build-mode repair)
+                                      result)
+                          rest)))
          (option '(#\s "system") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'system arg