Merge pull request #3129 from ilovezfs/fix-formula-prefix-envvar-for-head

build: fix HOMEBREW_FORMULA_PREFIX for head

Library/Homebrew/build.rb

@@ -112,6 +112,10 @@ class Build
       formula.extend(Debrew::Formula) if ARGV.debug?
 
       formula.brew do |_formula, staging|
+        # For head builds, HOMEBREW_FORMULA_PREFIX should include the commit,
+        # which is not known until after the formula has been staged.
+        ENV["HOMEBREW_FORMULA_PREFIX"] = formula.prefix
+
         staging.retain! if ARGV.keep_tmp?
         formula.patch