ikiwiki: Patch shebangs in plugins

Since the checkPhase runs before fixupPhase, we must manually patch
shebangs for plugins that are executed during tests.

Specifically, when docutilsSupport is enabled. The tests for the rst
plugin failed.

pkgs/applications/misc/ikiwiki/default.nix

···
     # State the gcc dependency, and make the cgi use our wrapper
     sed -i -e 's@$0@"'$out/bin/ikiwiki'"@' \
         -e "s@'cc'@'${stdenv.cc}/bin/gcc'@" IkiWiki/Wrapper.pm
+    # Without patched plugin shebangs, some tests like t/rst.t fail
+    # (with docutilsSupport enabled)
+    patchShebangs plugins/*
   '';
 
   configurePhase = "perl Makefile.PL PREFIX=$out";