An alias to "execute task-*" confuses task

Using `task` 2.6.0 (at commit 51a5a73c0, but also on the `master` branch), the following command:

```
task rc.alias.attach='execute task-attach' attach …
```

really confuses `task`, which seems to handle the `task-*` prefix specially. According to `strace`, the above results in the following call:

```
[pid 25304] execve("/bin/sh", ["sh", "-c", "- task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - execute task - attach 33"], 0x7ffd7da8e000 /* 56 vars */) = 0
```

Using `task_attach` works, as does `anything-else`.

Note that using `anything-doesnotexist` actually causes `task` to attempt to evaluate the expression:

```
~/.bin% task rc.alias.test='execute task-nonexistent' test
[task next ( - nonexistent )]
Cannot subtract strings
Configuration override rc.alias.test:execute task-nonexistent
~/.bin% task rc.alias.test='execute foo-nonexistent' test
sh: 1: foo: not found
Configuration override rc.alias.test:execute foo-nonexistent
```