set ROOT environment variable

[themancalledjakob]

fix linux install_dependencies.sh missing ROOT variable
#6542

[ofTheo]

Thanks @themancalledjakob !
Once the checks pass I'll merge this in.

[themancalledjakob]

Thanks @themancalledjakob !
Once the checks pass I'll merge this in.

great :) is there something i need to do to make them pass?

[ofTheo]

No - for some reason the travis and appveyor ones came back to life even though they are disabled in the repo settings.
As long as the Github Actions ones pass then it should be good to go. :)