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/compression.scm

@@ -2428,8 +2428,8 @@ at run time, and must be installed separately.")
                   (bin (string-append (assoc-ref %outputs "out") "/bin"))
                   (target (string-append bin "/" name))
                   (python (string-append (assoc-ref %build-inputs "python") "/bin"))
-                  (7z (string-append (assoc-ref %build-inputs "p7zip") "/bin/7z"))
-                  (unzip (string-append (assoc-ref %build-inputs "unzip") "/bin/unzip")))
+                  (7z (search-input-file %build-inputs "/bin/7z"))
+                  (unzip (search-input-file %build-inputs "/bin/unzip")))
              (setenv "PATH" (string-append (getenv "PATH") ":" python))
              (mkdir-p bin)
              (copy-file source target)