gzdev repository enable from GZDEV_USE_* files

[scpeters]

Initial attempt at gazebo-tooling/gzdev#65. Enable nightly / prerelease repository if .github/ci/GZDEV_USE_NIGHTLY or .github/ci/GZDEV_USE_PRERELEASE files are present. Needs testing.

Do we like GZDEV_USE_*? An alternative is to match the gzdev.py repository enable osrf nightly syntax like GZDEV_ENABLE_OSRF_NIGHTLY

[j-rivero]

Ouch sorry, the comments were more for changes in gzdev itself and this is modifying the code of CI on actions.

We probably want a solution that can be applied to both github actions and Jenkins, maybe better to change gzdev code itself to do the same for both?

Thanks for PR, I think it is a good starting point. Some thoughts before going forward:

* Where in the filesystem gzdev will look for the special configuration files? In an absolute path? Somewhere with respect to its current location?

* If there is a call to gzdev like gzdev.py repository enable --project=gz-math7, the configuration using the file is added to the existing in the central repo or the configuration is overridden?

Do we like GZDEV_USE_*? An alternative is to match the gzdev.py repository enable osrf nightly syntax like GZDEV_ENABLE_OSRF_NIGHTLY

Thinking on reuse this kind of files and given that the parameters to enable/disable repositories without declaring --project are positionals, we could try to split words and use them as parameters: GZDEV_WORD1_WORD2_WORD3 to do the same that gzdev word1 word2 word3 cli call. GZDEV_REPOSITORY_ENABLE_OSRF_NIGHTLY to be transformed by gzdev repository enable osrf nightly. Depending on the different gzdev modules we might want to restrict this to the repository module by now.