Run Rust shared library test via rustup to get the environment right.

test cases/rust/2 sharedlib/meson.build

@@ -2,4 +2,11 @@ project('rust shared library', 'rust')
 
 l = shared_library('stuff', 'stuff.rs', install : true)
 e = executable('prog', 'prog.rs', link_with : l, install : true)
-test('linktest', e)
+
+if build_machine.system() == 'windows'
+  rustup = find_program('rustup')
+  test('linktest', rustup,
+       args: ['run', 'stable', e])
+else
+  test('linktest', e)
+endif