install: fix HEAD installations with HOMEBREW_INSTALL_FROM_API

[Rylan12]

• 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 style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

This PR should fix the issues that HOMEBREW_INSTALL_FROM_API brought to HEAD formulae. Now, brew install --HEAD, brew outdated --fetch-HEAD, brew upgrade --fetch-HEAD should all work as expected.

CC @carlocab (if you could confirm that this fixes your issue I'd appreciate it, sorry I didn't get to fixing it earlier)

[BrewTestBot]

Review period will end on 2021-09-21 at 00:00:00 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Looks good thanks @Rylan12!

[Bo98]

alias_path is a String rather than a Pathname, so this can error in that scenario.

I'm tempted to make alias_path a Pathname but that would be an API change.

[Rylan12]

Suggested change

	default_specified_path = alias_path || path
	return default_specified_path if default_specified_path.presence&.exist?
	default_specified_path = Pathname(alias_path) || path
	return default_specified_path if default_specified_path.presence&.exist?

This will remove the error possibility, right?

[Bo98]

Pathname(nil) is an error unfortunately.