Add test for symlinking outside workspace

test/blackbox-tests/test-cases/github764/run.t

@@ -3,3 +3,4 @@
   [1]
   $ mkdir -p c2 && cd c2 && ln -s x ../../ && jbuilder build
   $ mkdir -p c3 && cd c3 && ln -s x y && ln -s y x && jbuilder build
+  $ cd symlink-outside-root && ln -s ../sample-exe sample && jbuilder exec --root . -- sample/foo.exe

test/blackbox-tests/test-cases/github764/sample-exe/dune

@@ -0,0 +1,2 @@
+(executable
+ ((name foo)))

test/blackbox-tests/test-cases/github764/sample-exe/foo.ml

@@ -0,0 +1 @@
+let () = print_endline "foo"