[service] Provide a global configuration setting accessible to recipes when a build is running on CCI

[planetmarshall]

What is your problem/feature request?

In #17509 @jwillikers suggested using a configuration variable to turn off certain options in the LLVM recipe except when it is running on CCI.

It would be usefult to have the inverse behaviour, that is, have a config variable available that can be checked to see if a build is running on CCI, then certain options such as resource limitations can be activated. However, users get the default behaviour when building locally.

See #17509 (comment)

[uilianries]

It would be usefult to have the inverse behaviour, that is, have a config variable available that can be checked to see if a build is running on CCI

@planetmarshall we have the environment variable CONAN_CENTER_BUILD_SERVICE=1. It's not documented because was added very recently.

[jcar87]

Please bear in mind that it's not intended for general use and will be only allowed on a case-by-case basis.
So please ask us before adding it to a recipe.

[uilianries]

I just created the PR #23052 to document the variable.

[planetmarshall]

Please bear in mind that it's not intended for general use and will be only allowed on a case-by-case basis. So please ask us before adding it to a recipe.

Added in d18fa46 for #22997