test: add shebangs to shell.nix test scripts

Fix intermittent SIGSEGV (exit code 139) on macOS when running
  nix-shell and shebang tests inside the nix sandbox.

  The foo, bar, and ruby test scripts were created without shebangs,
  which causes intermittent crashes when executed via command
  substitution on macOS. Adding proper shebangs resolves the flakiness.

  Potentially closes: #13106

(cherry picked from commit 7b3d7eb634)

tests/functional/shell.nix

@@ -96,14 +96,16 @@ let
 
     foo = runCommand "foo" { } ''
       mkdir -p $out/bin
-      echo 'echo ${fooContents}' > $out/bin/foo
+      echo '#!${shell}' > $out/bin/foo
+      echo 'echo ${fooContents}' >> $out/bin/foo
       chmod a+rx $out/bin/foo
       ln -s ${shell} $out/bin/bash
     '';
 
     bar = runCommand "bar" { } ''
       mkdir -p $out/bin
-      echo 'echo bar' > $out/bin/bar
+      echo '#!${shell}' > $out/bin/bar
+      echo 'echo bar' >> $out/bin/bar
       chmod a+rx $out/bin/bar
     '';
 
@@ -116,7 +118,8 @@ let
     # ruby "interpreter" that outputs "$@"
     ruby = runCommand "ruby" { } ''
       mkdir -p $out/bin
-      echo 'printf %s "$*"' > $out/bin/ruby
+      echo '#!${shell}' > $out/bin/ruby
+      echo 'printf %s "$*"' >> $out/bin/ruby
       chmod a+rx $out/bin/ruby
     '';