brew-services: don't hardcode launchctl path
brew-services: don't hardcode launchctl path

This change allows users to install the `launchctl` wrapper in the
`reattach-to-user-namespace` formula which makes the command work in
Tmux and other non-standard environments.

Closes Homebrew/homebrew#26609.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

mistydemeo/homebrew,MikeMcQuaid/brew,gregory-nisbet/brew,alyssais/brew,sjackman/homebrew,muellermartin/dist,Homebrew/brew,zmwangx/brew,DomT4/brew,rwhogg/brew,MikeMcQuaid/brew,muellermartin/dist,AnastasiaSulyagina/brew,nandub/brew,konqui/brew,Linuxbrew/brew,konqui/brew,claui/brew,nandub/brew,sjackman/homebrew,EricFromCanada/brew,jmsundar/brew,hanxue/linuxbrew,mahori/brew,ilovezfs/brew,palxex/brew,staticfloat/brew,aw1621107/brew,reelsense/brew,konqui/brew,amar-laksh/brew_sudo,zmwangx/brew,sjackman/homebrew,EricFromCanada/brew,jmsundar/brew,gordonmcshane/homebrew,palxex/brew,MikeMcQuaid/brew,Linuxbrew/brew,Homebrew/brew,tonyg/homebrew,staticfloat/brew,DomT4/brew,claui/brew,vitorgalvao/brew,hanxue/linuxbrew,MikeMcQuaid/brew,AnastasiaSulyagina/brew,maxim-belkin/brew,Homebrew/brew,JCount/brew,EricFromCanada/brew,Linuxbrew/brew,mgrimes/brew,amar-laksh/brew_sudo,jmsundar/brew,vitorgalvao/brew,nandub/brew,reitermarkus/brew,alyssais/brew,gordonmcshane/homebrew,gordonmcshane/homebrew,vitorgalvao/brew,ilovezfs/brew,ilovezfs/brew,mahori/brew,amar-laksh/brew_sudo,toonetown/homebrew,aw1621107/brew,Linuxbrew/brew,pseudocody/brew,pseudocody/brew,claui/brew,reitermarkus/brew,mahori/brew,tonyg/homebrew,mgrimes/brew,gregory-nisbet/brew,claui/brew,bfontaine/brew,JCount/brew,Homebrew/brew,mahori/brew,sjackman/homebrew,muellermartin/dist,hanxue/linuxbrew,zmwangx/brew,alyssais/brew,bfontaine/brew,bfontaine/brew,reitermarkus/brew,reelsense/brew,tonyg/homebrew,reitermarkus/brew,toonetown/homebrew,pseudocody/brew,mistydemeo/homebrew,DomT4/brew,EricFromCanada/brew,konqui/brew,rwhogg/brew,nandub/brew,vitorgalvao/brew,toonetown/homebrew,aw1621107/brew,mistydemeo/homebrew,AnastasiaSulyagina/brew,reelsense/brew,rwhogg/brew,gregory-nisbet/brew,JCount/brew,maxim-belkin/brew,staticfloat/brew,palxex/brew,JCount/brew,maxim-belkin/brew,DomT4/brew,mgrimes/brew