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/networking.scm

@@ -3701,8 +3701,9 @@ written (and providing API) in C.  Current implementation covers YANG 1.0 (RFC
       #:phases (modify-phases %standard-phases (delete 'configure))
       #:make-flags
       (list (string-append "PREFIX=" (assoc-ref %outputs "out"))
-            (string-append "PKG_CONFIG=" (assoc-ref %build-inputs "pkg-config")
-                           "/bin/pkg-config")
+            (string-append "PKG_CONFIG="
+                           (search-input-file %build-inputs
+                                              "/bin/pkg-config"))
             ,(string-append "CC=" (cc-for-target)))))
    (home-page "https://www.open-mesh.org/projects/batman-adv/wiki/Wiki")
    (synopsis "Management tool for the mesh networking BATMAN protocol")