post_install: use HOMEBREW_PATH.
post_install: use HOMEBREW_PATH.

Do this instead of appending the `HOMEBREW_PREFIX` as it's more
reliable at finding e.g. keg-only and requirement PATH additions. It's
not ideal as it overrides some environment filtering but the only real
alternative is recreate the entire build environment by shelling out to
`build.rb` in `formula_installer`.

Homebrew/brew,DomT4/brew,Homebrew/brew,reelsense/brew,alyssais/brew,konqui/brew,MikeMcQuaid/brew,claui/brew,reitermarkus/brew,sjackman/homebrew,pseudocody/brew,maxim-belkin/brew,reitermarkus/brew,EricFromCanada/brew,nandub/brew,sjackman/homebrew,JCount/brew,MikeMcQuaid/brew,alyssais/brew,mgrimes/brew,konqui/brew,aw1621107/brew,mahori/brew,nandub/brew,EricFromCanada/brew,mahori/brew,hanxue/linuxbrew,aw1621107/brew,JCount/brew,Linuxbrew/brew,claui/brew,reelsense/brew,mgrimes/brew,DomT4/brew,reitermarkus/brew,nandub/brew,vitorgalvao/brew,aw1621107/brew,pseudocody/brew,ilovezfs/brew,sjackman/homebrew,EricFromCanada/brew,reelsense/brew,Linuxbrew/brew,claui/brew,maxim-belkin/brew,DomT4/brew,MikeMcQuaid/brew,ilovezfs/brew,Linuxbrew/brew,pseudocody/brew,vitorgalvao/brew,Homebrew/brew,nandub/brew,JCount/brew,Linuxbrew/brew,mahori/brew,Homebrew/brew,konqui/brew,claui/brew,vitorgalvao/brew,hanxue/linuxbrew,vitorgalvao/brew,MikeMcQuaid/brew,JCount/brew,mahori/brew,alyssais/brew,reitermarkus/brew,EricFromCanada/brew,konqui/brew,mgrimes/brew,maxim-belkin/brew,DomT4/brew,hanxue/linuxbrew,ilovezfs/brew,sjackman/homebrew