gnu: Use 'search-input-file' some more.

This patch replaces occurrences of:

  (string-append (assoc-ref %build-inputs "…") "/…")

by:

  (search-input-file %build-inputs "/…")

* doc/guix.texi (Miscellaneous Services): Use 'search-input-file' in R
Shiny example.
* gnu/packages/admin.scm (screenfetch): Use 'search-input-file'.
(ufetch): Likewise.
(hosts): Likewise.
* gnu/packages/backup.scm (dirvish): Likewise.
* gnu/packages/code.scm (colormake): Likewise.
* gnu/packages/compression.scm (makeself-safeextract): Likewise.
* gnu/packages/debug.scm (scanmem): Likewise.
* gnu/packages/education.scm (snap): Likewise.
(omnitux): Likewise.
* gnu/packages/emacs-xyz.scm (epipe): Likewise.
* gnu/packages/games.scm (openttd-opensfx): Likewise.
(openttd-openmsx): Likewise.
(openrct2-title-sequences):  Likewise.
(openrct2-objects):  Likewise.
(mrrescue):  Likewise.
(0ad-data):  Likewise.
(xonotic-data):  Likewise.
(drascula):  Likewise.
(make-lure-package):  Likewise.
(make-queen-package):  Likewise.
(sky):  Likewise.
* gnu/packages/gnome.scm (network-manager): Likewise.
* gnu/packages/gnuzilla.scm (icedove): Likewise.
* gnu/packages/guile-xyz.scm (guile-shapefile): Likewise.
* gnu/packages/hurd.scm (netdde): Likewise.
* gnu/packages/javascript.scm (js-context-menu): Likewise.
(js-commander): Likewise.
(js-xmldom-sre): Likewise.
* gnu/packages/kde-frameworks.scm (krunner): Likewise.
* gnu/packages/kodi.scm (kodi-cli): Likewise.
* gnu/packages/libreoffice.scm (hunspell-dict-pl): Likewise.
* gnu/packages/linux.scm (e2fsck/static): Likewise.
* gnu/packages/markup.scm (markdown): Likewise.
* gnu/packages/maths.scm (hdf-java): Likewise.
* gnu/packages/multiprecision.scm (libtomcrypt): Likewise.
* gnu/packages/networking.scm (batctl): Likewise.
* gnu/packages/python-xyz.scm (python-pymediainfo): Likewise.
* gnu/packages/shells.scm (fish-foreign-env): Likewise.
* gnu/packages/tex.scm (texlive-fonts-iwona): Likewise.
* gnu/packages/upnp.scm (miniupnpc): Likewise.
* gnu/packages/version-control.scm (git-annex-remote-rclone): Likewise.
* gnu/packages/virtualization.scm (qemu): Likewise.
* gnu/packages/web.scm (icedtea-web): Likewise.
* gnu/packages/wm.scm (stumpish): Likewise.

gnu/packages/education.scm

@@ -337,17 +337,16 @@ easy.")
            ;; Replace the sole minified file in the package.
            (with-directory-excursion (string-append share "/src")
              (delete-file "FileSaver.min.js")
-             (symlink (string-append (assoc-ref %build-inputs "js-filesaver")
-                                     "/share/javascript/FileSaver.min.js")
+             (symlink (search-input-file %build-inputs
+                                         "/share/javascript/FileSaver.min.js")
                       "FileSaver.min.js"))
            ;; Create a "snap" executable.
            (let* ((bin (string-append out "/bin"))
                   (script (string-append bin "/snap"))
                   (snap (string-append share "/snap.html"))
-                  (bash (string-append (assoc-ref %build-inputs "bash")
-                                       "/bin/sh"))
-                  (xdg-open (string-append (assoc-ref %build-inputs "xdg-utils")
-                                           "/bin/xdg-open")))
+                  (bash (search-input-file %build-inputs "/bin/sh"))
+                  (xdg-open (search-input-file %build-inputs
+                                               "/bin/xdg-open")))
              (mkdir-p bin)
              (call-with-output-file script
                (lambda (port)
@@ -589,10 +588,9 @@ letters of the alphabet, spelling, eye-hand coordination, etc.")
                ;; Install the launcher.
                (let* ((bin (string-append out "/bin"))
                       (script (string-append bin "/omnitux"))
-                      (bash (string-append (assoc-ref %build-inputs "bash")
-                                           "/bin/bash"))
-                      (python (string-append (assoc-ref %build-inputs "python")
-                                             "/bin/python2")))
+                      (bash (search-input-file %build-inputs "/bin/bash"))
+                      (python (search-input-file %build-inputs
+                                                 "/bin/python2")))
                  (mkdir-p bin)
                  (with-output-to-file script
                    (lambda ()