internal/pkg: block on implementation entry

This avoids blocking while not in Cure method of the implementation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec_test.go

@@ -239,7 +239,9 @@ func TestExec(t *testing.T) {
 							cure: func(t *pkg.TContext) error {
 								return os.MkdirAll(t.GetWorkDir().String(), 0700)
 							},
-						}}, 1<<5 /* concurrent cache hits */), cure: func(f *pkg.FContext) error {
+						}}, 1<<5 /* concurrent cache hits */),
+
+						cure: func(f *pkg.FContext) error {
 							work := f.GetWorkDir()
 							if err := os.MkdirAll(work.String(), 0700); err != nil {
 								return err