build: fix HOMEBREW_FORMULA_PREFIX for head

[ilovezfs]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

---

so that it includes the commit.

This fixes the issue identified in Homebrew/homebrew-core#15950 (comment) where includes and libraries installed in the formula's prefix (e.g. resources installed in libexec) cannot be used during a build because superenv is using an incorrect value for HOMEBREW_FORMULA_PREFIX that doesn't include the commit, which causes it to filter out anything in the actual prefix.