Transform JRuby shebangs to absolute paths, fixing recursion within rbenv

The default shebang for binstubs in a JRuby installation is
`#!/usr/bin/env jruby`. On a rbenv-managed JRuby, this will execute the
`jruby` rbenv shim. In a situation where a JRuby tool like `gem` is
executed from within a running rbenv by a rbenv plugin, (e.g.
rbenv-gemset) this can trigger another rbenv run to resolve the shim.
This can lead to endless recursion of rbenv calls.

Fixes #471, closes #473
References jf/rbenv-gemset#56

bin/ruby-build

@@ -507,6 +507,7 @@ build_package_jruby() {
   ln -fs jruby ruby
   install_jruby_launcher
   remove_windows_files
+  fix_jruby_shebangs
 }
 
 install_jruby_launcher() {
@@ -515,6 +516,14 @@ install_jruby_launcher() {
   } >&4 2>&1
 }
 
+fix_jruby_shebangs() {
+  for file in "${PREFIX_PATH}/bin"/*; do
+    if [ "$(head -c 20 "$file")" = "#!/usr/bin/env jruby" ]; then
+      sed -i '' -E "1s:.+:#\!${PREFIX_PATH}/bin/jruby:" "$file"
+    fi
+  done
+}
+
 remove_windows_files() {
   cd "$PREFIX_PATH"
   rm -f bin/*.exe bin/*.dll bin/*.bat bin/jruby.sh